changing build system does not update build steps

[christian-rauch]

Changing between build systems (CatkinMake, CatkinTools, Colcon) in the build settings widget after the workspace has been initialised does not update the build steps.

Example:
A workspace that has been initially created with CatkinTools will contain a CatkinTools Step under Build Steps and Clean Steps. Changing the build system to Colcon will keep these build/clean steps, but disable them. Hence the workspace cannot be built anymore unless these steps are manually changed each to a Colcon Step.

For convenience, changing the build system should also update the build steps, keeping the previous steps disabled in case a user wants to copy&paste parameters.