Feature request: add a collapseIf feature to de.itemis.mps.editor.collapsible

Hello, I have an editor that contains a bunch of separate collapsibles. I would like to be able to toggle them all together (maybe via a boolean model element or button/checkbox). Right now I have to go manually to each collapsible and click on the +/- symbol to expand/collapse each one. It would be great if I could do it more programmatically along the lines of the showIf features on most cells. But in this case it would perhaps be a collapseIf function that returns a boolean and the collapsible cell responds accordingly.

Start by locating de.itemis.mps.editor.collapsible and the existing showIf features mentioned in the issue. Determine how a boolean callback or model-controlled value should affect collapsible state; done means separate collapsibles can be expanded or collapsed together programmatically.

Written by the indexing model from the issue text.