Avoid buttons getting "stuck down":

Avoid buttons getting "stuck down":
 - Simulate mouse button release if the area becomes disabled.
 - When a RelVerify button is pressed, set MUIA_Selected and MUIA_Pressed
   attributes on the button before (not after) updating MUIA_Timer, and
   check if the button has become disabled as a result of notifications
   on those attribute changes before starting the timer that runs while the
   button is pressed. An example of the problem this change solves was seen
   in the OpenURL prefs utility, where the buttons to move the positions of
   items within a list could get stuck in strange states, leading to items
   continuing to move after the mouse button had been released.
 - Refactoring: incorporated duplicated event-handler related code into
   release_button().

git-svn-id: https://svn.aros.org/svn/aros/trunk/AROS@51273 fb15a70f-31f2-0310-bbcc-cdcc74a49acc