Add external API with WindowMessages

* src/windowmessages.cpp: Handle new messages.
* src/windowmessagees.h: Add new codes.
* tests/Makefile.am: Update accordingly.
* tests/run-messenger.cpp: New.

The run-messenger can be used as an example how to use
this API. As we block saves while decrypted data is open
we add API to close the mails now. Either all or a specific
uuid.

e.g.:

run-messenger.exe 1301 1147d7ed-22fb-48d2-8678-a814df5009eb

• GnuPG-Bug-Id: T4241