use QHashCombine properly in qtdeclarative

$ cd qtdeclarative/src
$ git grep "uint qHash" | wc
    35 ...

so there are 35 qHash functions, and https://codereview.qt-project.org/c/qt/qtdeclarative/+/290645 is the first one that uses QHashCombine, which means most of them are doing ad-hoc arithmetic rather than using a shared function (or simple enough that they don't need to combine hashes).

https://stackoverflow.com/questions/2590677/how-do-i-combine-hash-values-in-c0x/54728293 sounds nice to have a variadic template solution too though, before we tackle this.