Blender Git Loki

Fix T45689: Pose Libraries cannot be used when using lib-linked actions as poselibs

This commit makes some tweaks that make it at least possible to use lib-linked
actions as Pose Libraries. Specifically:
* The apply poses button is no longer greyed out

* It is possible to select different poses from the list of poses

* All pose library operators which edit the poses stored in the poselib
now have improved poll callbacks which perform extra checks for lib-linked
actions (which cannot be edited, as all those changes will be lost)


Caveats:
* Due to the way the UI list template works, it doesn't seem to be possible to
make it not grey out the items in the list. (While the double-click to rename
thing shouldn't be allowed, items should at least look like they can be clicked on)

* The difference between clickable vs not-clickable isn't too great, making it hard
to tell that that while the Add/Remove/Sanitise toggles are not usable,
the Apply Poses is actually functional. But, this is a more of a UI-toolbox
level issue