Fix the look of the back button

and remove the grid is unnecessary.

https://bugzilla.gnome.org/show_bug.cgi?id=706943

Signed-off-by: Yosef Or Boczko <yoseforb@gmail.com>
mainToolbar: Use new close button in GtkHeaderBar

https://bugzilla.gnome.org/show_bug.cgi?id=707056