st-theme-node: Recalculate properties on stylesheet changes

As theme nodes keep a cache of matched properties, we need to make
sure to update it when the list of stylesheets changes. In particular
this fixes a regression from commit dc2ec0a8, which caused
extensions with stylesheets to crash the shell when re-enabled (for
instances when coming back from the lock screen).

https://bugzilla.gnome.org/show_bug.cgi?id=692994