Add map editor

[DevonPorcher]

Currently you can load a map from a text file. Implement a system to load this text file, and then be able to modify it visually. Then change it back a text file for later use.

[MadRubicant]

A note for anyone tackling this issue:
This is HARD. A map editor means you have to be able to load an arbitrary map from a file, make any number of (visual) edits to it, and when you're done, save it back to disk. This map then needs to be valid to use for the game itself. A fully-featured map editor is at least 500 points.

[MadRubicant]

F16 OK (500 pts)

[steve-fields]

F17 OK (500 pts)

[steve-fields]

W18 OK (500 pts)