Use a loading placeholder for project icons in the project manager

Now that projects are loaded asynchronously, some projects in the
list may be displayed before their icon is done loading. This is
especially common on slower hardware.

In such cases, this makes the project manager display a loading
placeholder instead of the default project icon.

editor/project_manager.cpp

@@ -1313,7 +1313,9 @@ void ProjectList::create_project_item_control(int p_index) {
 	hb->set_is_favorite(item.favorite);
 
 	TextureRect *tf = memnew(TextureRect);
-	tf->set_texture(get_icon("DefaultProjectIcon", "EditorIcons"));
+	// The project icon may not be loaded by the time the control is displayed,
+	// so use a loading placeholder.
+	tf->set_texture(get_icon("ProjectIconLoading", "EditorIcons"));
 	if (item.missing) {
 		tf->set_modulate(Color(1, 1, 1, 0.5));
 	}