[middleware/socket] Allow user to specify wished port

Fix #493
[doc] Document the 'port' keyword accepted by socket mw
[doc] Document in release note the new keyword 'port' for socket datastream
[sensor] Semantic Camera 'tag' and 'relative' properties

- 'tag' allows to select the kind of object visible
by the semantic camera
- 'relative' returns the visible objects position/orientation
in the camera frame

Fix #496
[doc] Document new features of semantic cam
[builder] Remove the VideoCamera::rotate overload

As discussed in #399.