When the St theme is changed, the StThemeContext unrefs all the theme
nodes cached in it's internal hash table, then emits a signal to
notify all theme nodes that the current theme has changed.

The problem is that the first StWidget to catch a theme changed signal
will trigger a "style-changed" signal catched by its children first.
So the theme changed signal can't be processed properly to cleanup
StThemeNodePaintState before recomputing the theme.

This patch adds a weak ref to the StThemeNode in the
StThemeNodePaintState to ensure paint states are properly cleaned up
when the associated StThemeNode is freed.

https://bugzilla.gnome.org/show_bug.cgi?id=703859