Add 'request_user_attention' to Window

This commit introduces a cross platform way to request a user attention
to the window via a 'request_user_attention' method on a Window struct.
This method is inspired by macOS's 'request_user_attention' method and
thus reuses its signature and semantics to some extent.

src/platform_impl/android/mod.rs

@@ -484,6 +484,8 @@ impl Window {
 
     pub fn set_ime_position(&self, _position: Position) {}
 
+    pub fn request_user_attention(&self, _request_type: Option<window::UserAttentionType>) {}
+
     pub fn set_cursor_icon(&self, _: window::CursorIcon) {}
 
     pub fn set_cursor_position(&self, _: Position) -> Result<(), error::ExternalError> {