LibGUI: Protect GUI::Button across firing the on_click hook

If a hook triggers the deletion of the GUI::Button, we would be unable
to proceed in a well-defined manner here, so let's protect ourselves.

This probably needs to be done in a whole lot of places, since GUI
widgets are just ref-counted Core::Objects and running arbitrary code
can mean that they get deleted.

Libraries/LibGUI/Button.cpp

@@ -108,6 +108,9 @@ void Button::click(unsigned modifiers)
 {
     if (!is_enabled())
         return;
+
+    NonnullRefPtr protector = *this;
+
     if (is_checkable()) {
         if (is_checked() && !is_uncheckable())
             return;