cssprovider: Names starting with -gtk- aren't style props

cssprovider: Names starting with -gtk- aren't style props

We want to have the "-gtk-" prefix for our custom CSS properties. But
we also want to parse names starting with a "-" as style properties.
So make sure that "-gtk-" is treated like a normal property and we emit
errors when somebody uses it wrong.

This is to catch errors with people typing
  -gtk-iconsource: none;
instead of the correct
  -gtk-icon-source: none;