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/Button.h

@@ -63,7 +63,7 @@ public:
 
     bool another_button_has_focus() const { return m_another_button_has_focus; }
 
-    void set_mimic_pressed(bool mimic_pressed);
+    void mimic_pressed();
 
     virtual Optional<UISize> calculated_min_size() const override;