documents: add first implementation of selection mode and osd toolbars
documents: add first implementation of selection mode and osd toolbars

This is namespaced to gnome-documents' style classes now, but it will
serve as a basis for a more generic style later.

ChrisCummins/gnome-themes-standard,rgcjonas/adwaita-semidark,DoctorJellyface/gnome-themes-standard,magcius/gnome-themes-standard,magcius/gnome-themes-standard,rgcjonas/adwaita-semidark,ChrisCummins/gnome-themes-standard,cosimoc/gnome-themes-standard,magcius/gnome-themes-standard,Distrotech/gnome-themes-standard,ChrisCummins/gnome-themes-standard,DoctorJellyface/gnome-themes-standard,cosimoc/gnome-themes-standard,ChrisCummins/gnome-themes-standard,cosimoc/gnome-themes-standard,ChrisCummins/gnome-themes-standard,Distrotech/gnome-themes-standard,rgcjonas/adwaita-semidark,cosimoc/gnome-themes-standard,rgcjonas/adwaita-semidark