display-page: Don't reveal topbar on pointer over edges

Seems all modern operating systems (GNOME, Windows 8, OS X) reserve
'pointer on edges' as gentures for different UIs. Lets try out best not to
interfer with that by only revealing overlaybar when pointer is around the
top center of the screen.

https://bugzilla.gnome.org/show_bug.cgi?id=674664
display-toolbar: Don't show WM decorations in fullscreen

Turns out that 'show-close-button' property is about all window
decorations and in classic-mode this means we show maximize and minimize
buttons and they seem misplaced in there in that context. We probably
don't even want to show 'close' button in fullscreen either. Hence
removing all window manager decorations.

https://bugzilla.gnome.org/show_bug.cgi?id=674664