More case creator staging (#2)

- Simplified some things in XmlReader/XmlWriter - rather than having a
billion #ifdef/#else statements, we now just typedef strings and images
differently depending on whether we're in game executable mode or case
creator mode.

- Added these new things to a common include file XmlIncludes.h, which
is included by both XmlWriter.h and XmlReader.h.

- Changed XmlWriter to be usable by the case creator, and also added a
mode in it to make its output human-readable, which will cause it to add
carriage returns and indents.