D2k: Add map importer

This is my initial attempt to create a map importer for Dune 2000 .MAP files. (#4281)
Current status:
 - Imports terrain
 - Imports resources (spice)
 - Imports actors (neutral, atreides and harkonnen; I will add the rest soon)

Issues:
 - Missing tiles from arrakis.yaml tileset file  (#5875)
 - Currently cannot add player references for mission maps

This is 90% working with skirmish maps (from what I've seen), excluding 10% for the missing tiles.
While this will import mission maps as well, the player reference issue does cause some problems there. This can be manually fixed in the `map.yaml` for now. I plan on fixing it properly when there is a Dune 2000 mission file reader implemented (#4280).
`class ImportLegacyMapCommand` that calls it is in `OpenRA.Mods.Common`.