We now use the GtkStleProviderPrivate interface, which hopefully is
faster and more conformant to CSS. Long term, it definitely should be
both.
I would have liked to split this up into multiple commits, but couldn't
find a way.
We used to only set the override color or font if it was different
than the existing value. However, that means you can't change it to
an override that is the same as the default value for the property.
With this fixed you can e.g. override with a color of 0,0,0,0 which
you couldn't before.
This object backs up gtk_widget_override_* operations. This object
is not meant to be public because any intention to modify widgets'
style in a themeable way should involve using regions/classes, so
they're modifiable through CSS. As such, the API is really
short-scoped.
