Bugfix [#27090] Available keying set fails in armature pose mode

Bugfix [#27090] Available keying set fails in armature pose mode

A change in the poll callback that Available KeyingSet used to use
restricted its use to Object-mode only, while this could also be
useful in Pose Mode (though it would only affect all channels there).

Made this use a custom poll callback now that tests for whether the
active object has an action. This does mean that if you select a bunch
of objects with animation data, but the active object doesn't have it,
then the keyingset will fail to fire, but that's been marked as a todo
in the code.