LibGUI: Rename Button method set_mimic_pressed => mimic_pressed

This function does more things than a simple getter, so it's probably
better to not use this notation.

I also drop the parameter as it was always used with the same value.

Userland/Libraries/LibGUI/Action.cpp

@@ -167,7 +167,7 @@ void Action::activate(Core::Object* activator)
 
     if (activator == nullptr) {
         for_each_toolbar_button([](auto& button) {
-            button.set_mimic_pressed(true);
+            button.mimic_pressed();
         });
     }