Use different offsets for windowmessages

* src/windowmessages.cpp (gpgol_window_proc)
(send_msg_to_ui_thread): Use WM_USER + 42
* src/windowmessages.h: Start enum at 1100.

While a better fix might be to use a nonce to ensure
that a message really originated from us I hope that this
is enough for other software that globally sends out
WM_USER messages.

• GnuPG-Bug-ID: T3769