Add axis= argument to BaseCamera.link
Add axis= argument to BaseCamera.link

The axis argument allows to confine linking between two PanZoomCamera's
to one axis only. View limits in this axis only will be kept in sync.

To implement this argument, get_state and set_state are extended to
understand tuple keys for selecting properties. These tuple keys select
subproperties in properties.

Eric89GXL/vispy,Eric89GXL/vispy,Eric89GXL/vispy