kcms/colors: Add an option to apply accent color from wallpaper

This adds an option to set the accent color from the current wallpaper.

The way it works is that it exposes some DBus calls; if a wallpaper plugin
supports wallpaper accent colors then it should say so by this DBus call
when starting up and also should say which wallpaper is being currently
used (for the purpose of extracting an accent color from it) or set an
accent color itself. After that it should keep declaring the wallpaper or
set accent color whenever wallpaper changes or whenever the plugin author
thinks it is appropriate to say so.

There is already an implementation of the color extracting algorithm which
the plugin can either use, or else set whatever accent color it wants. The
necessary DBus calls for the official image and wallpaper plugin are
implemented, so they should work out of the box.

BUG: 444676
FIXED-IN: 5.25