preferences: Set default_width to omit back button
The sizing of the preferences window depends on the button labels and
tends to be too narrow which makes the "back" button of
HdyPreferencesWindow
to be shown. It's a bit weird to see this on the
desktop when there's plenty of space available.
This is shown by HdyHeaderBar
when it considers a window to be
"mobile" which happens when its width is less than MOBILE_WINDOW_WIDTH
(400) or it's height less than MOBILE_WINDOW_HEIGHT (800) (see
libhandy/src/hdy-header-bar.c
).