Let Unlink Action operator have an undo step

I noticed while rigging a character and editing actions that the Unlink Action operator had no undo step. Doesn't feel intentional, so this patch adds the necessary flags.

Reviewed By: mont29

Differential Revision: https://developer.blender.org/D12346

Expose BLI_string_flip_side_name as bpy.utils.flip_name

Expose a new function in `bpy.utils.flip_name(name, strip_number=False)
that allows flipping bone names, eg "Bone.L" -> "Bone.R".

Useful for add-ons to avoid re-implementing Blender's name flipping.

Ref D12322

Armature Make/Clear Parent: Grey out options that don't do anything

In armature edit mode, the Make/Clear Parent operators don't do anything
in various cases, but only one of these cases was previously indicated,
and it was indicated by hiding the option completely instead of graying
it out.

Clear Parent (Alt+P) problems fixed:
- "Clear Parent" option always showed up, even when none of the selected
bones had a parent.
- "Disconnect Bone" option always showed up, even when use_connected on
all selected bones was already false.

Make Parent (Ctrl+P) problems fixed:
- "Keep Offset" option didn't show up when all selected bones' parent
was already the active bone. This was correct, and this patch tries to
make all behaviours consistent with this.
- "Connected" option always showed up, even when all selected bones'
parent was already the active bone, and they all had use_connect set
to True.

With this patch all options show up all the time, but in cases where
they would do nothing, they will be grayed out.

Reviewed By: sybren

Differential Revision: https://developer.blender.org/D6100

Custom Properties: support default values for strings

Differential Revision: https://developer.blender.org/D8102

Cleanup: Solidify modifier: Remove unneccessary error message.

For any modifier, the expected output when the input mesh is empty, is an
empty mesh. So this error message was useless, and could spam the
console in some usecases of the modifier stack...

Reviewed By: weasel, mont29

Differential Revision: https://developer.blender.org/D7571

Fix T65671: Armature X-Mirror inconsistencies

This fixes bendy bone properties not being mirrored correctly

Reviewed By: Brecht

Differential Revision: https://developer.blender.org/D5408