Switch to using variants for timestamps, split out signals

Switch to using variants for timestamps, split out signals

Like how we're handling activation, use GVariant for timestamps.  To
avoid polluting the GtkApplication API with GVariants, we rename the
GApplication signals to "quit-with-data" and "action-with-data".
GtkApplication will then wrap those as just "quit" and "action".

https://bugzilla.gnome.org/show_bug.cgi?id=621002