Update COPYING file

The current license for this component is out of date. Please see xfce4-taskmanager Issue #49 for additional details & discussion.

Briefly:

Comparing the 17 years old file with the new one, we have:

 - some whitespace changes
 - a new FSF address
 - some extra commas

All references to GNU Library General Public License have been replaced with GNU Lesser General Public License

cc: @aekoroglu