gtkprogressbar: fix size allocation

gtkprogressbar: fix size allocation

As of 74405cc, progress bars use a new design with values drawn on top
(or to the left) of the through instead of inside of it. This change
brought a number of regressions: the min-horizontal-bar-height and
min-vertical-bar-width style properties are not respected anymore. For
vertical progress bars, the value was drawn too close to the bar and not
centered vertically.

Fix this by respecting the style properties and drawing the value label
at the correct position.

Also, the xspacing and yspacing properties didn't server any apparent
purpose. Change their semantics to mean "the spacing between the label
and the bar". Hence, they only need to be added to the size request when
showing the label. Since we are changing semantics anyway, reduce their
default values from 7 to 2, to avoid and excessive gap.

https://bugzilla.gnome.org/show_bug.cgi?id=746688