Provide config interface for scripted effects and KWin scripts

A scripted component providing:
* ui/config.ui
* config/main.xml

can get a config interface by using the following in metadata.desktop:
X-KDE-ServiceTypes=KWin/Effect,KCModule
X-KDE-PluginKeyword=`X-KDE-PluginInfo-Name`
X-KDE-Library=kcm_kwin4_genericscripted
X-KDE-ParentComponents=`X-KDE-PluginInfo-Name`

`X-KDE-PluginInfo-Name` has to be replaced by the actual value. In case
of a KWin Script the X-KDE-ServiceTypes needs to be:
X-KDE-ServiceTypes=KWin/Script,KCModule

The GenericScriptedConfig tries to identify the package from the keyword
and creates a Plasma::ConfigLoader and loads the UI from the packaged UI
file.