Add camera rigs: add 2D camera rig

This rig is mostly useful for 2D shots, when the camera is static and
the action happens in front of it (like a theatre stage).

In a 2D production (and some shots in 3D as well), you sometimes need
to rotate the camera while zooming, effectively "cropping" the field,
just as you would using a rostrum camera. This is tedious and
error-prone if animating built-in basic transforms, so this rig
implements a more intuitive way to do that, by just animating the two
lower corners of the camera's field.

Also improved other stuff in the add-on:
- add the GPL license block to create_widgets.py;
- rename "arm[ature]" to "rig" in some functions, for consistency and
  to avoid confusion with the crane's arm;
- changes to the UI panel:
  - remove the boxes,
  - put focal length at the top of the panel,
  - group related properties using aligned columns,
  - change the Make Camera Active operator's poll method, so that it
    is always visible in the UI, but greyed out when the camera is
    already active.