Fix resource loading on Windows and standardize path normalization

* backslash to slash calls/procedures have been/should be replaced
  with GFileNormalizePath/u_GFileNormalizePath. I think I have
  replaced all instances.
* Resources requires the executable extension to be stripped on
  Windows, otherwise 'fontforge.*' resources are ignored.
  Symptoms include: The background not being white, colours not
  being obeyed etc.