[mc_rtc_gui] change the robot type to handle more informations of mc_rbdyn::Robot

[antodld]

This PR aims to provide more information of the mc_rbdyn::Robot through the mc_rtc::gui.
To do so, the PR adds a RobotMsg type handling all the info to be transmitted.

This will allow in the future to add more information about the mc_rbdyn::Robot through the mc_rtc::gui without the need to patch all gui clients repo.

However this update implies that all gui client using the robot module must be patched to handle the RobotMsg

[antodld]

The mc_rtc::gui::Robot type has been reverted to its original implementation and a new type mc_rtc::gui::RobotMsg has been added to handle more complex robot messages. The mc_rtc gui clients do not need to be updated directly anymore

[gergondet]

Thanks @antodld

I have rebased the commit on the master branch and I have made the following changes (I have kept everything in a single commit so this commit could be referenced in the future as a way to introduce a new element into the GUI)

• Append RobotMsg at the end of the Elements enum, this ensures that the type id does not change for the other elements (e.g. when updating mc_rtc on a robot but not on the terminal where the GUI is visualized)
• Moved RobotMsg to the mc_rtc::gui namespace as RobotMsgData
• Store a RobotMsg instance in the GUI object, otherwise rbd::paramToVector allocates on every call of the message building