General improvements, and Map-reading client code.

Fixed media player problems and improved several parts of the editor,
including the beginnings of a plugin API. Linked "Test" and "Build"
options to the export target (you no longer have to call
ExportSWF.test(), but it still loads most of the needed files manually
instead of from the project). Added tile assets, and began work loading
and displaying them in the haxe API.