Add support for comments/annotations within levels

[kevglass]

Similar to adding comments to code it would be nice to be able to add comments to levels, tile locations and entities. This could be used for documenting maps as they're being built or for including help tips for modders/mappers in sample maps.

[nyalloc]

Would be great if we could tag a comment as a "to-do" as well and have the editor list all to-do comments somewhere :)