mimeapps.list for plasma 6

[Harald Sitter]

In plasma 6 we are moving away from the KDE-specific InitialPreference
keys in desktop files [1] and instead install a mimeapps list [2] as
per [3]. You'll want to make sure that you install suitable
mimeapps.list(s) to set up things correctly for 3rd party apps such as
libreoffice.

[1] https://invent.kde.org/frameworks/kservice/-/merge_requests/181
[2] https://invent.kde.org/plasma/plasma-desktop/-/merge_requests/2030
[3] https://specifications.freedesktop.org/mime-apps-spec/mime-apps-spec-1.0.1.html

HS