Begin migration to Qt 5.2.
The QtPlugin architecture was changed, and as the current code does not reflect these changes, the extensions has been temporarily disconnected from build.

Prominent changes:
* Revert all QWeakPointers not pointing at Qt managed pointers to regular pointers
* QDesktopServices::storageLocation is now QStandardPaths::writableLocation
* QtMessageHandler (formerly QtMsgHandler) was updated

A lot of changes remains to be done.