Move debugger source path mapping to kit

It's very helpful at times to step through the Qt source code. Doing so requires downloading the source code and then mapping Qt Creator to the appropriate source folder. Example:

However, when I switch build kits (e.g., switching from 5.12.0 to 5.11.1), the source code still maps to whatever version it happened to be set to – in my case, it was still set to map to the 5.12.0 source code even though I switched to build with my 5.11.1.

This was jarring for me and resulted in a lot of wasted time while I debugged the wrong source code. It'd be nice if this sort of source mapping was a part of the Qt kits.