shortcut-editor: update reset button position and style

The Reset button, according to the mockups, should be placed
at the right side (left on rtl) of the shortcut label. Also,
rather than a plain string, the button should use a symbolic
icon for 'edit-clear' action.

This patch moves and updates the Reset button to match the
mockups.

https://bugzilla.gnome.org/show_bug.cgi?id=769314