Show which public companies can be started from each city

[ollybh]

It would be helpful to have an indication as to which public companies can be started using minor company tokens in each city.

• [x] When a minor tokens a city change the colours on the token.
• [x] Put a slot token in the open slots in cities at the start of the game.
• [ ] Use coloured tokens for ports and mines.
• [x] When cities are upgraded, add slot tokens to new slots.
• [ ] Remove all token colours and slot tokens after the stock round when the first public companies are started.