Add a fast path for parent selector matching that uses a bloom filter to
quickly discard selectors that can't possibly match.
Keep in mind that we match using a bloom filter, so we might
accidentally include too many selectors when hash/bucket collisions
occur.
That's not a correctness problem though, because we'll do a real check
afterwards.
The idea for this change is taken from browsers, in particular WebKit.
- GtkCssWidgetNode
for style contexts owned by a widget
- GtkCssPathNode
for style contexts using a GtkWidgetPath
- GtkCssTransientNode
for nodes created with gtk_style_context_save()/restore()
