New Quick Open Dialog

- Updated list view with thumbnails, and separate file name.
  - Added a grid view which has larger icons.
  - Added toggle to filter out files from addons.
  - Store history for each opened resource type.

New Editor settings for Quick Open:
  - Startup display mode (grid or list):
      - Determined by the requested resource type.
      - Whatever was last used.
  - Toggle to filter out files from addons (for persistence).

Notes
  - The dialog is now created once in EditorNode, and globally available for other components.
  - A fixed number of result scenes are instantiated, and reused based on query.
  - Drop support for multiselect.

editor/editor_resource_preview.cpp

@@ -414,6 +414,24 @@ void EditorResourcePreview::_update_thumbnail_sizes() {
 	}
 }
 
+EditorResourcePreview::PreviewItem EditorResourcePreview::get_resource_preview_if_available(const String &p_path) {
+	PreviewItem item;
+	{
+		MutexLock lock(preview_mutex);
+
+		HashMap<String, EditorResourcePreview::Item>::Iterator I = cache.find(p_path);
+		if (!I) {
+			return item;
+		}
+
+		EditorResourcePreview::Item &cached_item = I->value;
+		item.preview = cached_item.preview;
+		item.small_preview = cached_item.small_preview;
+	}
+	preview_sem.post();
+	return item;
+}
+
 void EditorResourcePreview::queue_edited_resource_preview(const Ref<Resource> &p_res, Object *p_receiver, const StringName &p_receiver_func, const Variant &p_userdata) {
 	ERR_FAIL_NULL(p_receiver);
 	ERR_FAIL_COND(!p_res.is_valid());