This patch adds choice for turntable/trackball in view3d orbiting (default via numpad).
It allows users to configure orbit style the same way as in pre-2.77 in User Preferences/Input.
New style from T47263 is let as default.
To mimic pre-2.77 behavior, users need to change Numpad-4 and Numpad-6 Style to Turntable in Preferences.
Setting all Numpad-2/4/6/8 to Turntable creates a more consistent behavior, i.e. the up/down rotation axis is really horizontal.
Corner cases are also handled.