Display a "loading" placeholder while icons are loading in the assetlib

editor/icons/icon_project_icon_loading.svg

@@ -0,0 +1 @@
+<svg height="64" viewBox="0 0 64 64" width="64" xmlns="http://www.w3.org/2000/svg"><path d="m8 0c-4.432 0-8 3.568-8 8v48c0 4.432 3.568 8 8 8h48c4.432 0 8-3.568 8-8v-48c0-4.432-3.568-8-8-8z" fill="#e0e0e0" fill-opacity=".188235"/></svg>

editor/plugins/asset_library_editor_plugin.cpp

@@ -65,7 +65,7 @@ void EditorAssetLibraryItem::_notification(int p_what) {
 
 	if (p_what == NOTIFICATION_ENTER_TREE) {
 
-		icon->set_normal_texture(get_icon("DefaultProjectIcon", "EditorIcons"));
+		icon->set_normal_texture(get_icon("ProjectIconLoading", "EditorIcons"));
 		category->add_color_override("font_color", Color(0.5, 0.5, 0.5));
 		author->add_color_override("font_color", Color(0.5, 0.5, 0.5));
 	}