FreeCAD 0.18 released with Python 3/Qt 5 support and more

More than 2500 commits. More than 100 contributors. More than 11 months in the making: https://www.freecadweb.org/downloads.php

0.18 is not a big change in terms of features but very important for the core. FreeCAD now supports Python 3 and QT5. Still, some important changes and convenience functions have been added in the user space as well: https://www.freecadweb.org/wiki/Release_notes_0.18