When loaded as in-process applet, we must not call `textdomain()` or we
will override mate-panel's text domain.  Instead, properly use
gi18n-lib.h header that provides the same API but uses the compile-time
`GETTEXT_PACKAGE` value for each call.

Actually drop the whole i18n setup logic that is provided already by
mate-panel's factory macros.

Fixes #219.

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.