This commit adds a position attribute for menu items defined in menu
templates. When the final menu is built the position attribute is used
to determine menu item positions in a similar design to how Eclipse
positions menu items.
In GTK+ radio menu items are managed automatically, so group id won't
have any effect there, in the meanwhile we need to maintain the same
behavior on all platforms, so we have to generate group id instead of
letting users specifying it.
