Don't regenerate shadows on every style-changed signal in StEntry/Label
Do a bit less work when we receive a style-changed signal in StLabel and StEntry by comparing the shadow specs and only invalidating the shadow pipeline in case it actually changed.
Since a bunch of other changes also affect the shadow (like changing the font), this MR starts listening to a bunch of ClutterText property changes and invalidates the shadow when things change.
Picking up changes to cursor position correctly depends on mutter!1757 (merged).
Edited by Jonas Dreßler