Changes

Bug 1249: NEWT/X11 Visibility: Listening to more events for updated
state; Adding QUIRK_BIT_VISIBILITY
1) More visibility detection on post ConfigureNotify events, since the
latter may not yet contain the updated visibility state as it whould
(WM bug!):
 - EnterNotify
 - LeaveNotify
 - Disabled
   - Expose
   - VisibilityNotify
2) Introducing quirks.
  Setting QUIRK_BIT_VISIBILITY to handle the issue where
  visibility -> false could not even be set.
 (commit: c61fa44)

WindowImpl.quirks: Access default -> protected, also fix API doc
comment.
 (commit: e794fc4)