[scripting] Support loading translations for UI loaded configs

Scripts and scripted effects can provide a ui file which gets loaded at
runtime. Obviously such a ui file is not translated. This introduces
quite a hack to load the translated strings.

In the metadata.desktop file the scripted component can provide a key
X-KWin-Config-TranslationDomain. The genericscriptedconfig tries to
locate the metadata.desktop file and reads this entry. If it is present
we extract all string properties of the loaded UI and pass them through
ki18nd.

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