main/postmarketos-ui-plasma-desktop: move some deps to extra

Move some dependencies from plasma-desktop to the -extras subpackage, so they don't get installed by default:

• audiocd-kio (2 MiB): using a CD drive with postmarketOS seems to be a very rare combination
• kolourpaint (3 MiB): very basic image editor with featureset of MS paint, I don't think it's that useful in a default install. Users looking for an image editor may want to use e.g. krita instead and can install whatever they want to use after the default install is done.
• kwrite (20 MiB): another text editor, kate (3 MiB), is already getting installed by default
• okular (81 MiB): quite big, and pdfs can also be viewed in firefox which we install by default
• mesa-demos (50 MiB): rather big for just being some graphical demos

It's always a good idea to keep the install image small, but in this instance the change was prompted because we currently hit a size limit when generating install images. In particular, when generating the v22.12 image for the pinebookpro. The process for generating install images is not optimized for size (see build.postmarketos.org#116) and this should be improved, but in the meantime let's just reduce the install size with this patch on edge and backport it to v22.12 so building this image doesn't fail anymore.

Sizes listed above are for aarch64 and include dependencies that get removed together with removing the package.