wayland: ensure bemenu is shown on top of fullscreen windows

Before this patch, wayland compositors that render fullscreen
windows between the top and overlay layers would show bemenu
behind an active fullscreen window. For other compositors
the z-rendering order was undefined.

This change ensures that bemenu will always be on top of other
usual windows (including fullscreen ones).

Fixes #81

lib/renderers/wayland/window.c

@@ -388,7 +388,8 @@ bm_wl_window_create(struct window *window, struct wl_display *display, struct wl
 {
     assert(window);
 
-    if (layer_shell && (window->layer_surface = zwlr_layer_shell_v1_get_layer_surface(layer_shell, surface, output, ZWLR_LAYER_SHELL_V1_LAYER_TOP, "menu"))) {
+    enum zwlr_layer_shell_v1_layer layer = ZWLR_LAYER_SHELL_V1_LAYER_OVERLAY;
+    if (layer_shell && (window->layer_surface = zwlr_layer_shell_v1_get_layer_surface(layer_shell, surface, output, layer, "menu"))) {
         zwlr_layer_surface_v1_add_listener(window->layer_surface, &layer_surface_listener, window);
         window->align_anchor = get_align_anchor(window->align);
         zwlr_layer_surface_v1_set_anchor(window->layer_surface, window->align_anchor);