inspector: Add missing directory when scanning for themes
modelbutton: Fix up for style context changes

- Don't do work anymore that's done in GtkButton
- Don't do a save/restore just to set the state, instead set the state
  in the right places.
Adwaita: use -gtk-icon-source: none on paned...

...to get rid of the handle decoration istead of setting a transparent
background and abuse a background image for the background color.