Blender Git Loki

Fix: Mouse presses in some areas do not set active modifier

There are a couple of operations that are meant to set the active
modifier that currently don't. The first is a mouse press on the drag
icon on the right of the header, and the second is mouse presses on
modifier sub-panels headers.

This was an oversight in the implementation, especially the second,
because the blank space on the right of a sub-panel header often looks
just like the blank space elsewhere on the modifier's panel that
*does* set the active modifier.

Note that this purposefully doesn't include collapsing and expanding
the modifier as operations that set the active, since regardless of
whether that makes sense, it wasn't in the agreed upon design, which
would ideally not need changing for 2.92.

Differential Revision: https://developer.blender.org/D10155