quartz: fix build on 10.6 and below

It is not possible to successfully build GTK+ on OS X 10.6 and below
since NSFullScreenWindowMask is only available starting with 10.7. Add
ifdef guards around setStyleMask: in order to allow it to build on
earlier OS X releases.

https://bugzilla.gnome.org/show_bug.cgi?id=737561
quartz: Move the FullscreenSavedGeometry definition earlier

Move the FullscreenSavedGeometry before the first (conditional)
use, to fix the build on OS X 10.6.

https://bugzilla.gnome.org/show_bug.cgi?id=737561
popover menus: Take iconic into account

We don't want separators in both side of an iconic section
and use a 10px margin to separate two iconic sections.

Separators are also updated in case of dynamic insertion
( often used with menu items for plugins )

https://bugzilla.gnome.org/show_bug.cgi?id=738650