Make SVGDIR a variable

Make SVGDIR a variable

Replace all occurrences of SVGDIR in the source by svgDir, a char array
that is initialized to SVGDIR, so that it can be altered at run time.
This is needed to make portable apps. Where SVGDIR was used in string
concatenation, the code had to be re-written to use snprintf to do the
concatenation at run time, through a new function LoadIconFile.
The extension of image files used there is now made dependent on the
__APPLE__ switch, so that it can use .png in OS X (which does not seem
to understand SVG icons).