Add qgltf support to qmake

qgltf is a tool provided by the Qt3D module that enables 3D assets to
be defined in qmake project files, and have them converted to an
efficient binary format at build time.  The qmake feature will convert
all 3D assets specified by the QT3D_MODELS variable to the qgltf
format and add the new model asset to the project as a Qt resource
file.

Change-Id: If7250d6f23a06254b1ed0e408057723763aad8c8
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>