Make window a GTK layer shell.

This change adds the gtk-layer-shell package and uses it to turn
windows built on linux into layer shells to allow for more
flexibility in their usage.

Cargo.toml

@@ -46,6 +46,9 @@ serde = { version = "1", optional = true, features = [ "serde_derive" ] }
 raw-window-handle = "0.5"
 bitflags = "1"
 crossbeam-channel = "0.5"
+gtk-layer-shell = "0.3.0"
+gdk-sys = "0.15.1"
+gdk = "0.15.4"
 
 [dev-dependencies]
 image = "0.24"

src/platform_impl/linux/window.rs

@@ -20,6 +20,8 @@ use raw_window_handle::{
   XlibWindowHandle,
 };
 
+use gtk_layer_shell;
+
 use crate::{
   dpi::{LogicalPosition, LogicalSize, PhysicalPosition, PhysicalSize, Position, Size},
   error::{ExternalError, NotSupportedError, OsError as RootOsError},
@@ -87,6 +89,9 @@ impl Window {
       .application(app)
       .accept_focus(attributes.focused)
       .build();
+
+    gtk_layer_shell::init_for_window(&window);
+
     let window_id = WindowId(window.id());
     event_loop_window_target
       .windows