Add support for the xkb-switch keyboard layout reader. (greshake#1386)

* Add support for the xkb-switch keyboard layout reader.

Closes greshake#1384 at greshake#1384

If you set the xkbmap layout to a multilingual string with variants, such as:

```
setxkbmap -layout us,es,ru
setxkbmap -variant dvorak,,phonetic
```

the `keyboard_layout` block will not show the current layout nor the current variant. It will only show the entire multilingual layout string. This is because `i3status-rust` uses the command [`setxkbmap -query`](https://github.com/greshake/i3status-rust/blob/b6e1b0b755de9f4520f1ecf17086d62fc5f190fe/src/blocks/keyboard_layout.rs#L63) to get the current layout.

```
$ setxkbmap -query                 ☸ 0007-04
rules:      evdev
model:      pc105
layout:     us,es,ru
variant:    dvorak,,phonetic
options:    grp:ctrls_toggle
```

This commit uses [`xkb-switch`](https://github.com/grwlf/xkb-switch) to read the current layout and keyboard variant.

Example usage:

```
[[block]]
block = "keyboard_layout"
driver = "xkbswitch"
format = "{layout} {variant}"
interval = 1
```

* Fix formatting and linter errors.

- Less newlines.
- Use unwrap_or_else instead of running unwrap_or with a function call.
  This improves efficiency by making the function call lazy.
- Use a character instead of a single-character string for split().

* Add documentation

doc/blocks.md

@@ -944,6 +944,7 @@ Four drivers are available:
 - `setxkbmap` which polls setxkbmap to get the current layout
 - `localebus` which can read asynchronous updates from the systemd `org.freedesktop.locale1` D-Bus path
 - `kbddbus` which uses [kbdd](https://github.com/qnikst/kbdd) to monitor per-window layout changes via DBus
+- `xkbswitch` which uses [xkb-switch](https://github.com/grwlf/xkb-switch) to show the current layout and variant. This works when `setxkbmap` is used to set a comma separated list of layouts, such as `us,es,fr`.
 - `sway` which can read asynchronous updates from the sway IPC
 
 Which of these methods is appropriate will depend on your system setup.
@@ -985,6 +986,17 @@ block = "keyboard_layout"
 driver = "kbddbus"
 ```
 
+Poll `xkb-switch` for current layout and variant:
+
+```toml
+[[block]]
+block = "keyboard_layout"
+driver = "xkbswitch"
+on_click = "xkb-switch -n"
+format = "{layout} {variant}"
+interval = 1
+```
+
 Listen to sway for changes:
 
 ```toml

src/blocks/keyboard_layout.rs

@@ -29,6 +29,7 @@ pub enum KeyboardLayoutDriver {
     SetXkbMap,
     LocaleBus,
     KbddBus,
+    XkbSwitch,
     Sway,
 }
 
@@ -411,6 +412,59 @@ impl KeyboardLayoutMonitor for Sway {
     }
 }
 
+pub struct XkbSwitch;
+
+impl XkbSwitch {
+    pub fn new() -> Result<XkbSwitch> {
+        Command::new("xkb-switch")
+            .output()
+            .block_error("keyboard_layout", "Failed to find xkb-switch in PATH")
+            .map(|_| XkbSwitch)
+    }
+}
+
+fn xkb_switch_show_layout_and_variant() -> Result<(String, Option<String>)> {
+    // This command should return a string like "layout(variant)" or "layout"
+    Command::new("xkb-switch")
+        .args(&["-p"])
+        .output()
+        .block_error("keyboard_layout", "Failed to execute `xkb-switch -p`.")
+        .and_then(|raw| {
+            String::from_utf8(raw.stdout).block_error("keyboard_layout", "Non-UTF8 input.")
+        })
+        .and_then(|layout_and_variant| {
+            let mut components = layout_and_variant.trim_end().split('(');
+
+            let layout = components
+                .next()
+                .ok_or("")
+                .block_error("keyboard_layout", "Unable to find keyboard layout")?
+                .to_string();
+
+            let variant = components
+                .last()
+                // Remove the trailing parenthesis ")"
+                .map(|variant_str| variant_str.split_at(variant_str.len() - 1).0.to_string());
+
+            Ok((layout, variant))
+        })
+}
+
+impl KeyboardLayoutMonitor for XkbSwitch {
+    fn keyboard_layout(&self) -> Result<String> {
+        xkb_switch_show_layout_and_variant().map(|layout_and_variant| layout_and_variant.0)
+    }
+
+    fn keyboard_variant(&self) -> Result<String> {
+        xkb_switch_show_layout_and_variant()
+            .map(|layout_and_variant| layout_and_variant.1.unwrap_or_else(|| "".into()))
+    }
+
+    fn must_poll(&self) -> bool {
+        true
+    }
+}
+
 #[derive(Deserialize, Debug, Clone)]
 #[serde(default, deny_unknown_fields)]
 pub struct KeyboardLayoutConfig {
@@ -468,6 +522,7 @@ impl ConfigBlock for KeyboardLayout {
                 monitor.monitor(id, send);
                 Box::new(monitor)
             }
+            KeyboardLayoutDriver::XkbSwitch => Box::new(XkbSwitch::new()?),
             KeyboardLayoutDriver::Sway => {
                 let monitor = Sway::new(block_config.sway_kb_identifier)?;
                 monitor.monitor(id, send);