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.

(I don't particularly like this, but we either do something like this or we revert the style-changed optimizations; all the style glitches on master are getting kinda annoying, so creating a MR now)

Fixes #1212 (closed)