LibGUI: Add an "exclusive" property to GAbstractButtons.

This makes checkable buttons exclusive with other checkable buttons in the
same parent widget. Basically like radio buttons. This should probably be
used to implement GRadioButton once it's a bit more mature.

LibGUI/GButton.cpp

@@ -60,6 +60,8 @@ void GButton::click()
 {
     if (!is_enabled())
         return;
+    if (is_checkable())
+        set_checked(!is_checked());
     if (on_click)
         on_click(*this);
 }