Add Ctrl+Shift+/ shortcut for the shortcuts dialog

Really what we want is Ctrl+? , which isn't necessarily
the same as Ctrl+Shift+/ depending on the user's keyboard layout.
The trigger system doesn't allow us to get around this right now
(and may not even be able to), see bug 62282.

Change-Id: I21353094be75e4a3bfcb5e693d18197657343e31
Merge "Add Ctrl+Shift+/ shortcut for the shortcuts dialog"