KApplicationTrader API

[David Edmundson]

>
> Note that the least-porting-effort solution is self() and trader language, since the code currently uses KServiceTypeTrader for this.
>
> But I'm tempted to say lambdas are nicer (you can debug the filtering more easily), and would allow to kill the whole
> trader-language-parser from kservice in KF6.

Looking briefly through the relevant Plasma code, lambda code would be
definitely do-able and potentially cleaner in some places.

There's some code in krunner that does a service type trader then does
another filter on top.
and notifications has a second iteration over retrieved results as
kservicetypetrader can't handle X-Flatpak-RenamedFrom

Plasma has 27 instances of KServiceTypeTrader::query("Application",
someConstraint) so not a crazy amount of work, especially considering
some are duplicates.

Given we can gradually port from KServiceTypeTrader and have years to
do it, I think it's worth it.

David