Take locked shape keys into account

[nickberckley]

Locking shape keys were introduced in Blender, it will be good to think about where to adhere to that, and do more tests.

• Duplicate/Split/Merge operators should still work if there are no bugs/glitches, since they're explicit actions taken by user.
• Locked shape keys shouldn't get keyframed with keyframe_all and action_bake operators. Can make a good workflow for choosing exactly what to keyframe.
• Similarly, locked shape keys shouldn't be taken into account when separating into separate objects. This will allow excluding some shape keys without needing to remove them.

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• In Merge operator, if any of the above/below shape keys are locked they shouldn't be merged with, or removed. This will enhance workflow and allow to choose what to merge very easily, without needing to reoder shape keys. Muted shape keys are merged but do not have any effect, so user could choose whether to: -- (a) Merge shape key and take it into account -- (b) Merge shape key but don't take it into account -- (c) Don't merge shape key

[nickberckley]

Locked keys should also be taken into account in Keymesh, during "Shape Keys to Keymesh" operator