Add to MouseArea new functions to emit messages containing the position on press and release events.

[IohannRabeson]

This PR allow to emit messages containing the cursor position when the user press and release mouse buttons in a MouseArea.

Currently you have to use MouseArea::on_move and cache the position received.
Now all you need is to declare a message that stores a Point.

Example of usage:

enum Message {
    Click(Point)
}

MouseArea::new(text("Hey"))
            .on_press_position(Message::Click)
            .into()