Bring Outlook back into focus after enc/sign

* src/mail.cpp (do_crypt): Send BRING_TO_FRONT message
after encryption.
* src/windowmessages.cpp, src/windowmessages.h: New call
to bring the active outlook window to front.

As explained in the comment this is a workaround for strange
behavior with pinentry and foreground windows. After encrypt/sign
often times the wrong window has focus. So we better bring
Outlook to front.

• GnuPG-Bug-Id: T3732