Blender Git Loki

NLA/AnimEditors: Added operator to remove all "empty" AnimData blocks

It is sometimes possible to end up with a lot of datablocks which have old + unused
"AnimData" containers still attached. This most commonly happens when doing motion
graphics work (i.e. when some linked-in objects may have previously been used to develop
a set of reusable assets), and is particularly distracting in the NLA Editor.

This commit adds an operator which removes AnimData blocks (restricted to only those
which are visible in the animation editor where it is run from) which are "empty"
(i.e. that is, have no active action, drivers, and nla tracks or strips).

This operator can be found from the "Edit" menu in the NLA Editor. Although it also
works when run from the DopeSheet or Graph Editors, it is of less use there since
those won't show these empty AnimData blocks by default (since by definition, such
AnimData blocks necesarily have no keyframes or drivers that can be shown), hence
there will be no feedback if the operator fails or succeeds.