mpsutil.intentions: add a new style attribute to allow intentions in read-only-cells

[alexanderpann]

The main idea is to still allow the intention menu even if nodes are, for example, just projected into the editor through a query list and read-only.

I've tried to add tests but couldn't find a way to test this. The invoke intention statement in the editor test doesn't check if the cell is read-only and trying to invoke a demo intention through actions or key presses also didn't work.