Use a style class for model buttons

Using a style class instead of the class name will let us vary model
button theming in the future.
Style the pane separator in the file chooser

https://bugzilla.gnome.org/show_bug.cgi?id=728358
Add a better fallback for file chooser when it doesn't use headerbar

https://bugzilla.gnome.org/show_bug.cgi?id=728358
Use the Adwaita icon theme
Remove cursors

These now live in adwiata-icon-theme.