Insight combo box selection fix (rebased onto develop)



This is the same as gh-784 but rebased onto develop.

----

Resolves a selection issue noticed in dev_4_4.

When viewing other users projects & datasets they were greyed out but you could still select them. This resolves that issue by using a combination of visual clues (via the renderer) and a combobox model that prevents the user from selecting items that are not viable.