Make service delay conditional on --service

Move registration of the D-Bus service from fr_application_startup to fr_application_command_line, and pass the value of arg_service as a boolean parameter.

This allows the registration to always execute, but avoid the g_application_hold and service timeout unless file-roller was launched with --service.

In my testing this seems to work fine, and prevents the slow exit from command-line invocations of file-roller. The D-Bus service can still be interacted with while file-roller is running, but it immediately closes down when the application exits (unless run with --service).

Fixes #34