allow to open multiple models from the same family table at once

[fmueller]

At the moment I can open one selected row (instance) of the family table via RMB or the "open" button.

This functionality should be also available when I select more than one row in order to allow all selected rows (instances) in separate windows at once.

That selected rows must not necessarily follow each other. I just want to select let's say the 2nd, the 4th and the 7th row and open them at once.