Add a daemon operational mode (#78)

Merged simplejack-src requested to merge simplejack-src/xfce4-appfinder:daemon_mode into master

New option called '--daemon', that when invoked causes the initial window to be created but not shown (creating alone seems enough to kick the idle work thread off). This is really only beneficial when running as a [D-Bus] service, as when '--disable-server' is also passed the main loop is skipped (no window, no service to tend, nothing to do) which resolves to a no-op.

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