build: Add Adwaita.css to EXTRA_DIST

Fixes: https://bugzilla.gnome.org/733011
build: Don't install thumbnail-frame.png, just EXTRA_DIST it

It is embedded into the application's binary using GResource. There is
no need to install it as an individual file.

Fallout from b09166df935d79c226fc2fa58bd2da2ffb243bf9
Prepare 3.13.4