Update license to LGPL 2.1.

Also update the preamble in source headers. Previously it referred to
the 'Lesser General Public License version 2', which is ambiguous. The
'Lesser' GPL is version 2.1. Version 2 is the 'Library' GPL.

The kdwsdl2cpp library which was imported from the kdepim repo
is left unchanged as 'LGPL2+'.