Added notification settings module + and mouseover fadeout to notifications.

This new settings module needs an icon.

I think the fadeout on mouseover highlights a bug in Cinnamon's CSS module: https://github.com/StealthOfKing/Cinnamon/commit/779ed40fdc2410fd78472e3f70ae65453b7c6c47#diff-9a16d9205ba2e1b1f10ee3c9fad12d86R669

I can't get a simple number value out of get_theme_node().

Depends: https://github.com/linuxmint/cinnamon-desktop/pull/9