Bring mail window to front after keyresolution

* src/common.c, src/common.h (bring_to_front): New.
* src/cryptcontroller.cpp (CryptController::resolve_keys): Use it.

Somehow when the Qt modal window is closed it brings the wrong
window back. This fix does not prevent that but at least returns
the previous active window to the front.

• GnuPG-Bug-Id: T3732