* Add 'invoke' style method, which can be used to call an interaction
from any thread. The interaction will be run in the appropriate
#GMainContext
* Sync methods can be called whether main loop is running or not.
* Derived classes can choose to implement only sync or async
interaction method, and the invoke method will fill in the blanks.
* Documentation for the above.
* Tests for the above.
https://bugzilla.gnome.org/show_bug.cgi?id=657567
