temp/gnome-clocks: add background mode and autostart

This is my attempt to add a background-mode to our "fork" of gnome-clocks, along with an autostart desktop file. The main driver is to allow alarms to survive a system restart without having to manually start GNOME Clocks again. This patch adds a -b / --background option to the gnome-clocks binary that will fully launch the process instance, but just not show the window. All subsequent instances will cause the window to be presented.