fzirob manage_underlays should re-source the workspace when changing underlays

[fmauch]

When changing the underlays it is obvious to me that I'll have to source my workspace in order to have my environment contain newly added underlays and open a new-shell if I want to get rid of environments. However, the "average user" might expect that the environment contains the correct workspaces immediately after changing the underlays...

This should probably get addressed when sourcing gets reworked.