Expose GContextSpecificSource so that things in GIO can use it directly.
Add support for a user_data area in the struct which will be helpful for
some intended use cases.
Also expose a way to emit a signal directly on a single source without
having to go through the group.
Add support for non-VOID__VOID signals to GContextSpecificGroup.
We keep the VOID__VOID case as a special optimised case by usual the
usual bit-stealing tricks.
Add a new internal helper called GContextSpecificGroup.
This is a mechanism for helping to maintain a group of context-specific
monitor objects (eg: GAppInfoMonitor, GUnixMountMonitor).
https://bugzilla.gnome.org/show_bug.cgi?id=742599
