No hardcoded QMLINSPECTOR_PATH in the qml app .pro file.

Instead of hardcoding a Qt Creator path into the standalone Qml
app .pro file by assigning a value to QMLINSPECTOR_PATH, we let
Qt Creator set that value as command line argument when calling
qmake.

QMakeStep::allArguments() is the place where it is added right
dt will will certainly suggest a better place place for it.