Add properties keywords & untranslatedKeywords to KPluginMetaData
Allows users of applications with bigger numbers of plugins to speed-up discovery of plugins by using those keywords in plugin view filters, e.g. KPluginWidget (see kcmutils!78 (closed))
While currently KCM plugins make use of the keywords, by the X-KDE-Keywords entry, the principle feature can be useful to any plugin set where the number is quite large and searching & filtering on finding a plugin can be helpful. It mirrors the generic Keywords field used in the desktop entry specification
Migrating from "X-KDE-Keywords" to "Keywords" will be a bit tricky, plugins will have to know they target systems build with support for "Keywords" only. So in case of Plasma that would be 5.25. Any other plugin system can start straight with simple "Keywords" name.