Fix: Changing default livery did not propagate to group liveries.

#11614 attempted to address this but did not handle 2CC properly, and changes to the default livery were not handled.

Feature: allow to do a hostile takeover of an AI company (in singleplayer) (#10914)

With the removal of the share-system, you could no longer make an
AI disappear in a single player game. At least, not without going
into the console.

Codechange: Make TileIndex a "strong" typedef to give it a distinct type.

This is accomplished by changing it to a single member struct with the
appropriate operator overloads to make it all work with not too much
source modifications.