install a kde-mimeapps.list

introducing a mimeapps.list has a number of advantages

- formalized encoding of default choices based on xdg tech
- non-kde apps will also respect this when inside a plasma session
- it paves the way to drop our initialpreference tech which has various
problems in a world where applications use any number of different xdg
implementations of which not all will be privy to the initialpreference

the list of defaults is seeded from the componentchooser kcm, plus
discover defaults used in neon already

(this file is part of plasma-desktop since mobile choices may be
divergent, making it a bad fit for plasma-workspace)

BUG: 305136


(cherry picked from commit 89c68cd7)