remove exit / replace with gtk_main_quit

[ 7590.265881] mate-notificati[76519]: segfault at 55ac5111c67a ip 00007d1d4fc09d61 sp 00007ffd0bf06698 error 4 in libgobject-2.0.so.0.8000.0[7d1d4fbdc000+37000] likely on  CPU 0 (core 0, socket 0)
[ 7590.265910] Code: 01 00 00 00 4c 89 ce 48 89 e5 e8 da e6 ff ff 5d 85 c0 0f 95 c0 0f b6 c0 c3 f3 0f 1e fa 48 85 ff 74 47 48 8b 07 48 85 c0 74 3f <48> 8b 00 48 3d fc 03 00 00 77 2c 48 8d 15 cd 53 02 00 48 c1 e8 02

Fixes #218.

This allows to run as a regular session service that does not exit, but
also as a well-behaved D-Bus-activated service.  Make the default
behavior not to exit, but when activated through D-Bus.