added a force reload command for live preview

Unbound Cmd-Shift-R on mac and bound the new command to Ctrl-Shift-R (Cmd-Shift-R on mac works automagically). This leaves the Debug > Reload Without Extensions command at Shift+F5 on all platforms (works on mac, too).

Putting this up for review - not sure what tests are needed (none are included right now).

This should resolve Issue #7667