Refactor accessibility for Qt5

Refactor accessibility for Qt5

* Moved most stuff to gui\accessible
* Moved widget-specific stuff to widgets\accessible
* Moved platform-specific code to either the bridge plugin (this was already the case)
  or to the platform plugin.
* Added several classes and functions. These have not yet gone through an API
  review. The plan is to do that in a later commit.
  Classes:
   - QPlatformAccessibility
   - QWindowsAccessibility
  Functions:
   - QWindow *QAccessibleInterface::window();
   - QPlatformAccessibility *QPlatformIntegration::accessibility()

* The bridge code can now either be a plugin or integrated into the platform plugin
* Mac accessibility is left out for now. Unix "should still work" (tm). These platforms
  should be fixed soon.

Change-Id: Ib49ffa73b647ee0af90864544c2769440157f562
Reviewed-on: http://codereview.qt-project.org/5330
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@nokia.com>
Reviewed-by: Jan-Arve Sæther <jan-arve.saether@nokia.com>