Support Python Launcher (py.exe) to detect Python

Both qtwebengine and qtdeclarative tries to find a valid Python 2 from the environment. qtdeclarative just checks for "python", while qtwebengine also checks e.g. for "python2".

Anyhow, on Windows Python since a while installs a Python Launcher, py.exe. Much like qtchooser, this allows users to switch between Python versions without having to tamper with PATH's.

Both qtdeclarative and qtwebengine should therefore be extended to check for py.exe -2 too.