Blender Git Loki

AnimEditors UI: Move less frequently used filtering options to a "Filters" popovers

This commit is an experiment in using popovers as a place to house some
of the filtering options for animation editors, in line with what's taking
place in the Outliner with the filtering popover there too.

Right now, the most frequently used/changed options are still available
on the headers (i.e. the "Only Selected"/"Hidden"/search fields), while
everything else (i.e. the per-datablock filters, which were already hidden
behind a collapsed-toggle button before) now live in the popover.