Border for "separator_menu_item" should be 0

Summary:
This revision aims to fix a crash in Sequencer64 (and possibly other apps) with this message:

```
(seq64:25899): WARNING **: 09:40:24.716: Invalid borders specified for theme pixmap:
        /usr/share/themes/Breeze/gtk-2.0/../assets/line-h.png,
borders don't fit within the image
```

BUG: 404045

Test Plan: No visual changes nor crashes in other apps.

Reviewers: #breeze, mthw, gepardo, jackg, ngraham

Reviewed By: #breeze, mthw, ngraham

Subscribers: ngraham, zzag, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D21869