De-duplicate metadata for QML KPackages
This way the embedded json metadata of the C++ plugins will be used instead.
By moving the main.qml file to a "ui"-subfolder we don't need to configure the paths for the mainscript, but can just use the defaults.
To test this change, all the KCMs packages in "PREFIX/share/kpackage/kcms/" should be removed. Otherwise we might have some leftovers lying around.