Legend for 'operator' map type isn't particularly useful

[MarcellPerger1]

The legend for the 'operator' map type might be better if it showed the colors for the operators currently on-screen rather than 'railway line' and '(unknown)'

[hiddewie]

This issue should have been solved by completing issue #597, and related pull requests #707, #716, #717, #718, #720 and #735.

There are now 331 known operators in the list.