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)