trivial: st-widget: Remove super old 'stylable' property

I don't think we ever used this, even way back in 3.0...

https://bugzilla.gnome.org/show_bug.cgi?id=644306
trivial: Fix the signedness of boolean fields

The standard old kludge with gboolean being signed, not unsigned.
Encountered while printing the values for debugging.

https://bugzilla.gnome.org/show_bug.cgi?id=644306