cssselector: Handle region flags during parse

cssselector: Handle region flags during parse

When parsing a selector like
  tab:active
We used to create 2 selector objects, one for the region, and one for
the pseudoclass. Now, for convenience, we special-case region
pseudoclass parsing so that the active flag is added to the region
selector.

A side effect is that ":nth-child(1)" no longer works for regions.
Hopefully people didn't use this but used the saner ":first-child"
instead.