new action from selected NLA channnels

when baking an armature animation that has animated custom properties, the properties can't be baked. They can be kept by choosing to overwrite current action when baking, but after that it's not possible to bake again without losing them.

To resolve this and other similar scenarios regarding baking of properties, it would be helpful to be able to select the custom properties channels and then click a channel menu item named 'move to new/existing action'.

This would save a lot of manual labour and potential loss of work for those unfamiliar with the peculiar behavour of properties being unbakeable.