implement tip popup tip selection and change default brush color to black

src/app.cpp

@@ -568,7 +568,7 @@ void App::terminate()
     TextureManager::invalidate();
     ShaderManager::invalidate();
     layout.clear_context();
-    brushes->clear_context();
+    //brushes->clear_context();
     layers->clear_context();
     color->clear_context();
     stroke->clear_context();