Fix GimpMenu separator lines being randomly thick
As reported by @Wormnest in !837 (merged):
When right clicking in the image area to get the main menu, the divider lines are a lot thicker and uneven, compared to the thin lines when you access the menu from the top.
I have a first patch in a stash for this, but I'll need to clean it (or possibly redo it differently, unsure).
Also this issue appears in the top menu too when I use it inside a GtkHeaderBar instead of the main app menu through GtkApplication API.