change QtVersionManager::instance() return type to QtVersionManager*

Enough time has passed to make developers acquainted to the static
methods of QtVersionManager. Returning the correct type in the instance
method has the following advantages:
  - signal/slot autocompletion is working again
  - Qt5's type-safe connect syntax becomes usable without casting to
    QtVersionManager*

Change-Id: Ib20c0ba5a8b0152c403330a4463f50e64578f44a
Reviewed-by: hjk <hjk121@nokiamail.com>