shortcut-editor: ensure sensitive entries on creation mode

When creating a new shortcut, we currently assume the entries are
sensitive and just show the dialog. This, however, may not be, for
example after previously canceling the editing of a custom shortcut,
leading to a state where the name and command entries are insensitive.

Fix that by always making sure the entries are sensitive when
setting the dialog to creation mode.

https://bugzilla.gnome.org/show_bug.cgi?id=777824