Fix Enter causes page reload in shortcuts editor

[parente]

Currently:

1. Open keyboard shortcuts editor
2. Click to change / assign a keyboard combination
3. Type the keys to assign
4. Press the Enter key to save the assignment
5. Entire browser page refreshes

I think the return false in the current code was meant to stop this from happening, but a call to event.preventDefault() is also required to prevent full-page form submission.

I think the behavior could still use some tweaking since pressing Enter in the dialog even when a shortcut field does not have the edit focus does nothing.