After having implemented our own configuration-writing script, the project doesn't use kwriteconfig5 directly anymore, which yields the previous sentence incorrect. This fixes that.
Moves the way theming is done from the workspace module, changing from a pure file-based method to using plasma CLI-tools like `plasma-apply-colorscheme` and similar tools. Also adds options for wallpaper, look-and-feel, cursortheme and icontheme. Also rename colorscheme option to colorScheme
