fix the absolute path to wxwidgets294 in vcxproj

>- "treesheets" contains the Visual Studio 2010 files for treesheets, open the .sln.
  If you've done the above correctly, ~~TreeSheets will now compile and pick up
  the wxWidgets libraries~~.

Following your instructions, I couldn't build TreeSheets. Then I found that you didn't use relative paths for the wxWidgets libraries. I made some changes that fixed the issue. You may find them useful.