st/theme-node: Forget properties cache on stylesheet change

st/theme-node: Forget properties cache on stylesheet change

To avoid `node->properties` pointing to freed memory after each
`st_theme_unload_stylesheet`, flush the cache that is `node->properties`.
They will be reloaded as soon as they are required by `ensure_properties`.

And yes `node->stylesheets_changed_id` already existed, but was unused.

Why not just fix Croco? Croco does not use proper reference counting
everywhere it should, and retrofitting it would be difficult due to the
recursive nature of `CRDeclaration`.

Includes commit:
st/theme-node: Forget properties cache before unref'ing the theme
Which the properties point into.

Forwarded: https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3252
Bug: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/7339
Bug-Ubuntu: https://bugs.launchpad.net/bugs/2069559
Bug-Dash-to-Dock: https://github.com/micheleg/dash-to-dock/issues/2179

Gbp-Pq: Name st-theme-node-Forget-properties-cache-on-stylesheet-chang.patch