Added trackpad gesture support for Qt + lots of new documentation

Ginga gets trackpad gestures!  The Qt rendering class gets support for
pinch and pan gestures:

* The pinch/rotate gesture works as expected on a Mac trackpad
* The pan gesture is not a two-finger pan but a "non-standard", Qt-specific
  one-finger pan.  These are experimental for now, but are enabled by
  default in this release.

Also in this release there has been a lot of updates to the
documentation.  The developer and internals sections in particular have
a lot of new material.