st: Consider non-background properties for ::style-changed

st: Consider non-background properties for ::style-changed

st_theme_node_paint_equal() was originally added to preserve paint state
when a style change didn't affect any of StWidget's cached background
resources.
That's why using it for filtering out unneeded style changes as in commit
f662864a misses any non-background related properties that are relevant
for subclasses. Add additional tests to make sure we keep emitting the
signal in those cases.

https://gitlab.gnome.org/GNOME/gnome-shell/issues/1212