Renaming Plasma's applications.menu file

[Nicolas Fella]

Hi,

In KF5 KService ships /etc/xdg/menus/applications.menu, our menu
definition according to
https://specifications.freedesktop.org/menu-spec/menu-spec-1.0.html

Since this will cause a conflict between KF5 and KF6 (and potentially
other desktop environments) for Plasma 6 I'm planning to:

- Move the file to plasma-workspace

- Rename it to plasma-applications.menu

- Set XDG_MENU_PREFIX="plasma-" at Plasma startup

See https://phabricator.kde.org/T12542,
https://invent.kde.org/frameworks/kservice/-/merge_requests/153, and
https://invent.kde.org/plasma/plasma-workspace/-/merge_requests/3053

@distributions: I know some distributions change the name of this file
downstream (i.e. via -DAPPLICATIONS_MENU_NAME=foo) to work around
aforementioned issues. Such workarounds should no longer be needed and
removed.

Cheers

Nico