Blender Git Loki

Fix Action Editor unlink button when in tweak mode

When in NLA tweak mode, the action unlink button in the Dopesheet /
Action Editor should be a mere shortcut to exiting tweak mode [nothing
else].

Instead, it was also clearing the action fully, not returning to the
previous edited action before going into tweak mode.

Now dont "flush" by clearing the action, instead exit tweakmode, clear
the scenes SCE_NLA_EDIT_ON flag (if this isnt done some NLA operators
like pushdown were not immediately available because their poll checked
this flag) and send appropriate notifier to have everything update nicely.

Part of T87681 (Bug 4/5/6).

Maniphest Tasks: T87681

Differential Revision: https://developer.blender.org/D11052