This introduces a cmake variable in KNewStuffCore which contains a KDEInstallDirs based location for our knsrc files, and further adds functionality to fetch search locations from KNewStuff.
A fallback is currently included, as we have to be able to work with older software. We also unfortunately cannot properly mark anything as deprecated, as there previously was no API for this, but rather a basic instruction to just install knsrc files in the system configuration directory (instructions which previously not even a part of the API docs, which this patch also addresses). The code was written with future deprecation in mind, and we can at that time simply remove the configuration fallback options and the functionality it holds.
Rationale:
This change is being made on request from various distributions, as the current install location for knsrc files is causing a great many issues during the distribution update cycle which means we are left with stale knsrc files in various places. This in turn causes a great many issues, particularly for Discover which has suffered crashes and other strange side effects when attempting to use these stale files (some of which attempt to access servers which almost-but-not-quite respond). Discover has workarounds for some of these, but it is becoming more than a little awkward, and being able to manage this a little more tightly would come in very handy indeed.