Use the ki18n and kdoctools cmake macros to install the translated content, so we can properly:
- ship and install the GUI messages (i.e.
- ship and install the translated documentations, which so far haven't been shipped at all
This also rework the in-source layout of the plugin translations, still installing them as before.
A global change is that there is no more 80% completion threshold for translations: all the shipped translations are compiled and installed. As I wrote in the message of the last commit: this is something that has not been generally done for many years in KDE, as the completion percentage does not say much about the actual usefulness of a translation. If something like this is needed again, then a better way is to simply not copy translations in the sources, rather than ignoring them.