screen: Allow getting the cmdline of the current foreground process

This would further enhance our ability to search based on the current
foreground process.

https://bugzilla.gnome.org/show_bug.cgi?id=711075
app: screen: Add API to map UUIDs to screens

https://bugzilla.gnome.org/show_bug.cgi?id=711075
server: Add a search provider

https://bugzilla.gnome.org/show_bug.cgi?id=711075