Use the incoming/outgoing capabilities for trimming loaded plugins

Only lets the plugins that support something offered on the other end or
that offer something supported on the other end. If we don't have
information about the other device, we enable everything like we used to.

Also refactors a bit the plugin loader, to be able to provide information
step by step.

Currently it works, but we need support for capabilities at least on
Android to start taking good advantage of it.

REVIEW: 118820
CCMAIL: kdeconnect@kde.org