Allow parameters to have a display name

[krixkrix]

I suggest parameters should have an optional display_name attribute.

Currently the name attribute is the most suitable text to display in a user interface, but often this means showing very code-near names like motorx_speed.

I would like to be able to give a display name for the parameter, e.g. Motor X speed [mm/s]

I don't want to use the description attribute for this since it serves another purpose of longer, descriptive text.

[mikaelarguedas]

Having a display_name is a good idea!

The approach in #112 sounds good.

Some things to take into account / modify:
even if the default value of the field in the .msg file is an empty string, this changes the message definition, the md5sum of the msg definition and likely the tcp packets, so we will have to:

• define rosbag migration rules so that existing messages can be translated to the new message
• wait for ROS Noetic before rolling out the new feature