RFE - Re-enable changing map tilesets for single-theme maps.

[SimonLandmine]

As a result of a number of maps being built with mixed-theme maps, the tileset-selection option was disabled.

The suggestion is that, if the map selected in the Lobby is parsed as only using a single theme, the option to select alternative themes is enabled (in the Lobby), so that e.g. a grass-only theme map can be converted to desert for all players.

It is suggested that this takes place in the Lobby, so that it can be transmitted to the other players so that all of the players are playing on the same theme.

[SimonLandmine]

@kuronekochomusuke - This is not something that would happen in the Map Editor, but in the Lobby. In the Map Editor, you can already have all the tileset theme fun that you want.
It would require the Lobby to parse the map and identify whether it only uses a single theme. If that is the case, it would then enable an option to change the theme (e.g. switch an 'all-desert' map to 'all-snow'). This option should not be available if the map was built using multiple themes.