Cloning Bug #295836 since they are actually two different bugs.
This one is referred to the API to set and get the favourite handler,
There is a review on reviewboard that needs to be fixed in order to support requested=true/false and target=single/group here: https://git.reviewboard.kde.org/r/107295/

+++ This bug was initially created as a clone of Bug #295836 +++

I was thinking about some API in common-internals, that instead of 
having them as constant, make them configurable, defaulting to our 
components or to any other installed handler if our is not installed.
Even though there is no way to configure it (the only way is to edit 
manually the config file), the approver partially does that (see 
ktp-approver/src/handlewithcaller.cpp:38), we should move that in common 
and use it in every component (it would be great if we could do this in 
time for 0.4)