Improve fullscreen behavior

Follow HIG for fullscreen: https://developer.gnome.org/hig/stable/fullscreen.html.en

Hide window buttons, instead introduce a restore button.

As a result of discussion in #gnome-design, also hide new tab button, because tabs are inaccessible.

Also add .titlebar class to the headerbar to have solid background on elementary. I've no idea if that's what's wanted, but at least it's legible. removed that, it breaks incognito style in funny ways.

Fixes #938 (closed)