gapplication: add a way to override the app-id

Some applications support running in a mode where they present
themselves as a different application to the user (for example web
browsers or terminals).

To facilitate this, add an option --gapplication-app-id which allows
users to override an application's id from desktop files or similar.

Applications need to opt-in to this by setting the
G_APPLICATION_CAN_OVERRIDE_APP_ID flag.

https://bugzilla.gnome.org/show_bug.cgi?id=743933