Fix .desktop file to adhere to modern FDO standards.

--- a/dist/python-whiteboard.desktop
+++ b/dist/python-whiteboard.desktop
@@ -3,7 +3,7 @@
 Type=Application
 Encoding=UTF-8
 Name=python-whiteboard
-Icon=pywb_pixmap.xpm
+Icon=pywb_pixmap
 Exec=/usr/bin/python-whiteboard
-Categories=Application;Education
+Categories=Education