Cleanup: document and simplify `BKE_animdata_set_action()`

Simplify `BKE_animdata_set_action()` by flipping some conditions and using
early returns, and document which parameters can be `NULL`.

No functional changes.

Cleanup: animation, deduplicate "can edit action" logic

Move "action editable" check from RNA code to Blender kernel
`BKE_animdata_action_editable()`.

No functional changes.

Cleanup: animation, remove commented-out code

The code was commented out in rB6307c65534f6d5cf29d08220cea0dbcca9a0e118,
11 years ago.

No functional changes.

Cleanup: animation, reduce complexity of RNA update function

Reduce complexity of `rna_SpaceDopeSheetEditor_action_update()` by flipping
conditions and returning early.

The depsgraph tagging has slightly changed, in that the depsgraph is not
tagged at all when there is no actual animation data added. I still see
this as a non-functional change, though, as in that case nothing changed
and tagging and re-evaluating wouldn't make any actual difference.

No functional changes.

Animation: set Action `idroot` at assignment instead of just at evaluation

Actions are either locked to a specific ID type, or "floating". Actions
in the floating state are now locked when they are assigned to an ID block.
Previously (rB94b99b5d4a7c20cf2) this was done at evaluation time, which
caused various problems:
- The ID type was set on the evaluated copy, and inconsistently flushed
back to the original.
- A newly created Action could not be assigned to an Action constraint,
unless a depsgraph evaluation was be forced first.

This is now resolved by calling `BKE_animdata_set_action()` to set the
action (instead of direct assignment) where possible, and calling
`BKE_animdata_action_ensure_idroot()` otherwise.

Manifest Task: https://developer.blender.org/T80986

Fix T80874: Actions inside NLA Strips are not copied to buffer/append

Restore NLA Action expansion code that was accidentally removed in
rBbed634c4f96.

Cleanup: NLA, refactor condition, use early return

Simplify code by replacing `if (strip) { everything; }` with
`if (strip == NULL) { return; }`.

No functional changes.

NLA: remove tagging copy-on-write when tagging recalc-animation

Remove `ID_RECALC_COPY_ON_WRITE` when tagging `ID_RECALC_ANIMATION`; the
former is unnecessary when using the latter.

No functional changes.

Fix T80708: FCurve modifiers change behavior after editing NLA clip

Tag an Action for recalculating animation when it's pushed down onto the
NLA stack, as its effective boundaries change. Normally an Action spans
all of the timeline, and the effects of Cycle modifiers are visible
everywhere. When the Action is converted to an NLA clip, the modifier's
effect should only be visible for the duration of the NLA clip.

Fix: re-do versioning code for FCurve extremes and Collection colors

Bump Blender subversion from 4 to 5, and re-execute some versioning code.

Because of a mistake in the versioning code (corrected in rB1a4fc6dcd67b),
some of the versioning code didn't run in certain cases. This subversion
bump ensures that this code is still run.