kicker: don't crash on ksycoca database changes

as part of database changes we call AppEntry::reload which calls into KService which may implicitly refresh the database if the data changed again meanwhile. This leads to further signals getting emitted and now reaching us in an undefined state, eventually crashing.

queue the signals through the eventloop instead

SENTRY: PLASMA-WORKSPACE-5HC SENTRY: PLASMA-WORKSPACE-6P5