Mark disallowed schema definitions based on the model selection

[pomek]

The new Schema tab (#155) shows all schema entries registered in an editor, which significantly improves understanding of the model limitations. Thanks for adding it.

I believe we can improve the list by marking (e.g., gray out) items disallowed in the given context based on the selection, similar to the Commands view.

So, for the given model:

The imageBlock definition will be grayed out as the editor does not allow adding of block images in captions.

[oleq]

Pros:

• Aligned with the commands pane that also displays whether a command is enabled or not.

Cons:

• Could be a source of confusion because the user may not be able to connect the state of the Schema pane to the Model pane they don't see. They may be confused why some definitions are "available" and some are not (greyed out). 
  • Commands also react to other editor states (e.g. read only) so the analogy to the Commands pane may not be the best.
  • OTOH, they may eventually figure this out by moving the selection.