1) The "pushdown" operation is the actual name for the functionality
previously represented by the snowflake. That is, pushing the active
action onto the NLA stack as a new strip.
The icon that is introduced here is the icon I originally wanted to
use here for this purpose (but couldn't at the time the NLA was coded
as we didn't have the master SVG available yet). I'm making this change
now to hopefully make the "animation-layers" intentions behind the NLA
design a lot clearer than they have been so far.
2) Also added a dedicated icon for representing that the active action
is in the "tweaking" state (i.e. we're editing a particular strip's
action). Previously we were abusing an icon designed for an entirely
different purpose, mainly since its identifier looked like it would work.
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