scripting: Refactor generic scripted config

Currently it makes an assumption that the name of a scripted effect
starts with "kwin4_effect_" prefix, which has its issues. For example,
it will stop working if we choose to use simpler naming scheme.

This change reworks how the generic scripting kcmodule is constructed.
Instead of guessing kpackage type, make relevant kcms pass it explicitly
via the arg list.