Gio: Add some TODOs about changing the base classes.

	* gio/src/actionmap.hg
	* gio/src/pollableinputstream.hg
	* gio/src/pollableoutputstream.hg
	* gio/src/remoteactiongroup.hg:
	We should probably derive from their prerequisite
	(required) classes instead. These were added in
	glibmm 2.34.