[Bug]: Scenario editor: Many random towns function creates towns with duplicate names

[JGRennison]

Version of OpenTTD

master

Expected result

All town names are unique

Actual result

Repeated use of the many random towns function in the scenario editor may create towns with duplicate names.

This is particularly the case when using poorly implemented town name generators, such as the French one.

GenerateTowns only checks the set of towns being added are unique with respect to each other, not with towns already on the map.

Steps to reproduce

Repeatedly use the many random towns function in the scenario editor

Villes de France.scn.zip