Support 3 options:
* VIRCADIA_USE_SYSTEM_QT is set -- use system's Qt
* VIRCADIA_QT_PATH is set -- use Qt found in that dir
* Otherwise -- use downloaded Qt package
This removes these variables:
* VIRCADIA_USE_QT_VERSION
* VIRCADIA_USE_PREBUILT_QT
* HIFI_QT_BASE