PixelPaint: Use layer menu as context menu in LayerListWidget

This enables the layer menu as a context menu in LayerListWidget,
setting the clicked layer as active for now, but in the future it
would be nice to have custom menu applying to the clicked layer instead
of the active layer.

Userland/Applications/PixelPaint/main.cpp

@@ -405,6 +405,10 @@ int main(int argc, char** argv)
         },
         window));
 
+    layer_list_widget.on_context_menu_request = [&](auto& event) {
+        layer_menu.popup(event.screen_position());
+    };
+
     auto& filter_menu = menubar->add_menu("&Filter");
     auto& spatial_filters_menu = filter_menu.add_submenu("&Spatial");