Resolve "Extract/Compress menu items are hidden if extension menu items present"

"Extract Here", "Extract To…" and "Compress…" menu items are contained by the extensions menu section, despite not being an extension.

This has worked fine until the extensions menu handling has been reworked in commit bd81bd89

This commit introduced a regressions which causes the Extract/Compress items to disappear if actual extension menu items are added.

Since it was already conceptually wrong to have them in this section, let's move these items into their own menu section to fix the bug.

Fixes #1472 (closed)