Blender Git Loki

Fix: Incorrect submodes being set in Dopesheet/Action Editor when changing modes

This commit fixes a number of problematic corner cases when switching between
editors after 2b5050a4cdfbb075d360fd39433acea07432c60b

The root cause of these issues was that mode_prev was not being set in
many cases, resulting in mode changes to the Timeline and back (via other
editors) causing Dopesheet Editors to reset back to "Action Editor" mode.

1) Creating new Dopesheet Editors (e.g. change the default 3D view to
a Dopesheet editor) would default to displaying the "Action Editor",
since mode/mode_prev == 0 represents the "Action Editor" (for backwards
compatability reasons), while mode == 3 is for the "Dopesheet"

2) If you set the Dopesheet Editor to another mode (e.g. "Grease Pencil" mode),
change to another editor (e.g. Shaders), then come back, the mode would
get reset to "Action Editor".