Qt5/KF5 and extending registration of plugins (formerly done with more desktop files)

[Friedrich W. H. Kossebau]

Hi,

in the old times, with metadata about plugins separated in .desktop files, one 
could register the same plugin for multiple things without touching the plugin 
itself. 

Examples:
plugins that were using plugin themselves, like the Okular kpart. When 
installing another Okular plugin, e.g. for document type X, one would simply 
install another desktop file, saying
--- 8< ---
[Desktop Entry]
Icon=okular
Name=Okular
X-KDE-ServiceTypes=KParts/ReadOnlyPart
X-KDE-Library=okularpart
Type=Service
MimeType=X;
--- 8< ---
et voilà, the plugin module would also be registered for X files. Works 
because the Okular kpart internally uses another lookup mechanism to find its 
own correct plugin for that type X.

Any idea how this principle can and should be mapped into the modern Qt5/KF5 
times, where the (initial) metadata is directly injected into the plugin? How 
could 3rd-party extend the metadata now?

Cheers
Friedrich