Add off-screen hot buffer indicators in buffer list

Indicators are located at the top and the bottom of the buffer list,
touching window insents, if any. They are shown if there's a hot buffer
above or below the visible buffers, including buffers that are partially
visible. When pressed, they cause the buffer list to scroll to the next hot
buffer in the direction of the button, trying to center the buffer.

The indicators are image buttons with background that is transparent while
the button is not pressed. Both the image and the background are rounded
rectangles. The image is a combination of the badge that we use showing the
number of highlights for a buffer in the buffer list, and the double arrow
that we use for jumping to bottom in the chat.

app/src/main/java/com/ubergeek42/WeechatAndroid/adapters/BufferListAdapter.kt

@@ -25,7 +25,7 @@ import androidx.recyclerview.widget.DiffUtil
 import androidx.recyclerview.widget.RecyclerView
 import androidx.recyclerview.widget.RecyclerView.ViewHolder
 import com.ubergeek42.WeechatAndroid.R
-import com.ubergeek42.WeechatAndroid.adapters.BufferListAdapter.*
+import com.ubergeek42.WeechatAndroid.adapters.BufferListAdapter.VisualBuffer
 import com.ubergeek42.WeechatAndroid.databinding.BufferlistItemBinding
 import com.ubergeek42.WeechatAndroid.relay.Buffer
 import com.ubergeek42.WeechatAndroid.relay.BufferList
@@ -35,7 +35,7 @@ import com.ubergeek42.WeechatAndroid.upload.main
 import com.ubergeek42.WeechatAndroid.utils.Utils
 import com.ubergeek42.cats.Kitty
 import com.ubergeek42.cats.Root
-import java.util.*
+import java.util.Collections
 
 
 class BufferListAdapter(
@@ -206,6 +206,14 @@ class BufferListAdapter(
         }
     }
 
+    //////////////////////////////////////////////////////////////////////////////////////////// Etc
+
+    @MainThread fun findNextHotBufferPositionOrNull(positionsToSearch: IntProgression) =
+        positionsToSearch.firstOrNull { position ->
+            val buffer = buffers.getOrElse(position) { return@firstOrNull false }
+            buffer.highlights > 0 || (buffer.type == BufferSpec.Type.Private && buffer.unreads != 0)
+        }
+
     companion object {
         @Root private val kitty: Kitty = Kitty.make()