This commit exposes the "Push Down" button/functionality found in the NLA Editor
to the Action Editor, so that actions can be added NLA Stack from here too. The
main point of this for now is to make the whole layered-animation workflow nicer
more efficient, but not requiring the second editor be visible in common cases.
It also conveniently sets things up for the next few changes (already hinted at
here)...
b28a24091f