GtkLabel: Rely on the new pango support for context change tracking

GtkLabel: Rely on the new pango support for context change tracking

Now that Pango tracks changes to the context automatically there is
no need to do it manually in e.g. style-updated or direction-changed,
in fact the only case we have to care about is when we re-create
the PangoContext due to a screen change, so we only have to clear
the layouts in GtkLabel in screen-changed.

This means we're not clearing all the layouts whenever the state changes,
which happens to every widget when the window is unfocused, which helps
performance a lot.

https://bugzilla.gnome.org/show_bug.cgi?id=340066