keyboard: switch to the reset button

Per the mockups [1], when editing a custom shortcut, there is
a reset button right after the current shortcut. When there
is no shortcut set, it shows a "Set Shortcut" button instead.
The current UI to edit a shortcut, however, diverges from
the proposed one since we use an "Edit" button instead.

Fix that by swapping the Edit button with a reset button, and
adapt the code to not depend on the edit button anymore.

[1] https://raw.githubusercontent.com/gnome-design-team/gnome-mockups/master/system-settings/keyboard/keyboard-wires.png

https://bugzilla.gnome.org/show_bug.cgi?id=777845