Add application-specific theme for Adwaita

This patch adds some desktop style when the Adwaita theme is used. Adwaita is the default theme of GTK+ since version 3.14, so it would be worth if Nemo support it.

Based on this patch for Nautilus:
https://git.gnome.org/browse/nautilus/commit/?id=3fd7e847d531fbbfc4ebba24864bdd0f8b81c750