qt: Use same focus indication for labels as Kleopatra

* qt/focusframe.cpp, qt/focusframe.h, qt/keyboardfocusindication.cpp,
qt/keyboardfocusindication.h: New.
* qt/Makefile.am: Add new files.
* qt/main.cpp (main): Add KeyboardFocusIndication to the application.

With this change keyboard focus of labels is indicated with a focus frame
(or whatever the used widget style draws as QStyle::PE_FrameFocusRect).

• GnuPG-bug-id: T5863