themes: Restore default progressbar min sizes

themes: Restore default progressbar min sizes

The min size on the oriented axis used to come from style props with
default values in the source file, used if the theme did not provide a
min size in CSS. When the style props were removed, so was any notion of
a minimal size for proressbars' main axis, meaning that now progressbars
without expand or any other source of min size were just tiny specks.

The right place to do that was always the theme, so in our themes now,
fix that by copying the old default values for the style properties; see:

https://gitlab.gnome.org/GNOME/gtk/issues/1191#note_259393
https://gitlab.gnome.org/GNOME/gtk/blob/gtk-3-24/gtk/gtkprogressbar.c#L92

The result should be the same in that (A) the min size is now what it is
in GTK+ 3 & (B) an app/user can override the theme exactly the same way.

Close https://gitlab.gnome.org/GNOME/gtk/issues/1192