Move font DPI syncing to kcminit

StartPlasma's fontDPI syncs the current font DPI to xrdb, which affects
font DPI for X applications (inc apps run in xwayland)

This is seldom used on wayland as with framebuffer scaling you don't
want to adjust your font DPI, but it is used by some users. As it's
xrdb, it needs to come after Xwayland is started for the wayland case.

The goal of this patch is to move towards killing the
startplasma-waylandsession binary as well as just being better design to
have these things comparmentalised.

kcminit is very early in the boot process before all GUI apps with the
exception of ksplash. I don't think this will make a big difference as
that's just a picture anyway?