Map Editor: Add "Start of scenario" text

It'd be nice if maps could contain a brief introduction at the start, for map makers who are sharing maps around.

It'd be useful for setting up story-lines with the maps, writing in specific challenges the map is intended for, or simply allowing the map maker to sign their work so people can find more of their stuff.

I believe it'd be a quick and easy 'low hanging fruit' type option for helping to build the community around the game. I personally have a 'campaign' in mind, involving a dozen maps with specific intended challenges and a mini story about a family of beavers setting out to realize a dream.