garcon-menu-separator: Fix constructor

This partially reverts 1045e3f0, where the static qualifier was removed without reason and without making the rest of the function consistent with this change. The GarconMenuSeparator is not intended to be packed in a container, so it can effectively be created once and then only referenced.

This also fixes a memory leak reported by Valgrind on the weak reference added on separator, which did not make sense anymore with a non-static variable.

Related: !5 (merged)