Swapping ASCII font generator for bitmap-based

Makes the "Generate ASCII tiles" function request a bitmap file to work
with, and populate the tileset with bitmap ASCII from that bitmap
instead of font-generated ASCII.
Also fixes the Main() path in build.ant and provides a build.xml so that
Ant can be just started from the directory.

TODO: Check that the tilepage matches the selected tileset dimensions
TODO: Revert to font-based ASCII generation if no tilepage is selected