This is far less code and allows for an easier enforcing of standards, for
example the name of the log identifiers which were adjusted in a few cases.
Also clean up unused includes when noticed.
In theory we support Qt 5.6 (because of SailfishOS) but in practice we are not checking that in the CI so probably we broke the compatibility long ago. Also, I don't think anyone is using this code in SailfishOS, so we can greatly simplify the code by requiring Qt 5.15 or later.
Uses KPluginLoader+KPluginMetadata instead.
Describes plugins in json instead of desktop files. These desktop files are
then embedded into the .so file. All the plugins will be in a kdeconnect/
directory, and those will be the ones to look for.
Note it doesn't drop the KService dependency as KIOCore is a KDE Connect
dependency as well.
REVIEW: 123042
