Set input as handled when processing shortcuts in BaseButton.

This fixes the issue when the script editor tab is active and the scene tree editor is focused, a ctrl+a press both opens the new Node popup and selects all text in the script editor.
Doesn't seem to have negative side effects so far.

scene/gui/base_button.cpp

@@ -358,6 +358,7 @@ void BaseButton::_unhandled_input(Ref<InputEvent> p_event) {
 		}
 
 		on_action_event(p_event);
+		get_tree()->set_input_as_handled();
 	}
 }