Load Map button is disabled until the editor scans the maps directory and maps have been found. Closes issue #1.

- StartState will now scan the directory for any files within it. If any are found, each is checked for a "<map>" tag which contains a version string matching the map driver version in the code.
- StartState will draw at least one frame before it begins to scan the maps directory so that the user can see something is, in fact, happening.
- StartState will draw one frame while beginning to load a map so that the user can see that something is, in fact, happening.
- Windows message boxes have been reactivated to display error messages when exceptions are thrown.