cds: enable outset shadows

Adds conditional code paths to GdkCssShadowValue for painting outset
shadows, and allows shadows to be applied in two passes (first outset
then inset). This can be used to draw csd shadows in outer window
borders.

https://bugzilla.gnome.org/show_bug.cgi?id=695998
Update GtkCssProvider box-shadow documentation

Non-inset box-shadows are no longer ignored by the GTK+ theming engine,
outset shadows are supported.