st: Ensure to reset all widget theme nodes

Theme node invalidation stops at unmapped widgets, and widgets
that forget to chain up to the default ::style-changed implementation.
This may leave stale nodes that were invalidated on
StThemeContext::changed, but are still set on widgets, and maybe
used for CSS property lookups.

Make sure that theme node invalidation happens always by moving
propagation outside the vfunc, and ensure the theme nodes are reset
across the full actor tree. Emission of ::style-changed, and obtaining
a new theme node may be delayed till when the actor is mapped.

The theme node is also cleared after unparenting an actor to avoid
stale references.

This ensures that all widgets get their theme node cleared after
stylesheet changes, instead of maybe being left with a theme node
that's been cleared of all its properties.

Fixes: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/2541

https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1223

(cherry picked from commit 325ff73c)