feat: Add readonly for skeleton

[ArneGudermann]

resolves #1100

[sveneberth]

I rather imagine a readOnly state at skeleton level here.

So on the one hand an attribute that can be set at skeleton level.
Then the value is inherited when rendering (but not set in the bone), so that you can deactivate the readOnly state again and know which bones were originally readonly.
And the readonly should then also prevent a toDB. And possibly this should also be mentioned when rendering in the renderer. (maybe this could be interesting later for cases where an edit is not possible but a view is).