Separate config files according to version

[chapulina]

Desired behavior

We place GUI and server configuration files under ~/.ignition/gazebo, as explained on these tutorials:

• https://ignitionrobotics.org/api/gazebo/5.0/gui_config.html
• https://ignitionrobotics.org/api/gazebo/5.0/server_config.html

The problem is that these files may differ according to Gazebo's version, and if the user has more than one version installed, they may try to load an unsupported config. Ideally, the files should be installed in versioned folders, i.e.:

• ~/.ignition/gazebo/3/gui.config
• ~/.ignition/gazebo/4/server.config

And Gazebo should only save and load files from the current version's folder.

Alternatives considered

Instead of version numbers, we could have collection names, i.e. citadel, dome.

Implementation suggestion

It would be good to do this for stable releases while keeping backwards compatibility. For example, Gazebo could always look for ~/.ignition/gazebo/gui.config first, and use that if it exists.

[chapulina]

Placing the files in a versioned directory from Fortress: #1050

Still need to think more about how to fix this nicely for stable releases.