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.