qmlls: go to definition of C++ defined types

Implement go to definition for C++ defined types. We only have the header name from the qmltypes file inside the qqmljsscopes, so we need a heuristic to find the actual header path from the header file name.

One idea could be to use the paths from QQmlCodeModel::rootUrls() (if they are correctly populated) and run `find` or similar to find files with the same name. I am not sure if we need to differentiate between files with similar names, maybe we could make sure that the name of the QML component appears somewhere in the file.