Revert "Fix return type for EditorResourcePreview::check_for_invalidation"

This reverts commit 716dfa67a3c5a0a38205179c01676e05a718d244, since it was later
fixes differently with c6b55e45e9256bb41b2d045d8c61d1946de881b1...

tools/editor/editor_resource_preview.cpp

@@ -336,7 +336,7 @@ void EditorResourcePreview::_bind_methods() {
 	ADD_SIGNAL(MethodInfo("preview_invalidated",PropertyInfo(Variant::STRING,"path")));
 }
 
-void EditorResourcePreview::check_for_invalidation(const String& p_path) {
+bool EditorResourcePreview::check_for_invalidation(const String& p_path) {
 
 	preview_mutex->lock();
 

tools/editor/editor_resource_preview.h

@@ -121,7 +121,7 @@ public:
 	void queue_edited_resource_preview(const Ref<Resource>& p_path, Object* p_receiver, const StringName& p_receiver_func, const Variant& p_userdata);
 
 	void add_preview_generator(const Ref<EditorResourcePreviewGenerator>& p_generator);
-	void check_for_invalidation(const String& p_path);
+	bool check_for_invalidation(const String& p_path);
 
 	EditorResourcePreview();
 	~EditorResourcePreview();