ProcessDialog: Make "back" buttons icon-only on desktop too
ProcessDialog: Make "back" buttons icon-only on desktop too

It looks silly when you go forward in some process and the icon-only
"close" button changes into a textual "back" button. Additionally, in
WikimediaUI theme, the textual labels look very poor combined with the
faux-frameless styling for these buttons.

Bug: T225021
Bug: T226045
Change-Id: Id466452c6ffd51794fac49745ed45767fc7abb93

wikimedia/oojs-ui,wikimedia/oojs-ui,wikimedia/oojs-ui,wikimedia/oojs-ui