This patch adds Qwt.

For the license, I picked LGPL2.1, but in fact Qwt has its own “Qwt 
License”, which is the same as the LGPL2.1, but relaxes (or tries to 
relax) some of its conditions (see 
http://qwt.sourceforge.net/qwtlicense.html).  Can we stick with LGPL 
for our purposes?

I had to disable building of the qtcreator plugin, because this tries 
to link against the qwt library in the build directory, which is not 
accepted by the guix ld-wrapper (at least that's my understanding of 
it).  Suggestions on how to fix this are welcome (though at the 
moment, qtcreator is not packaged for guix anyway?).

Also, I put this in qt.scm, but perhaps it should go in a separate 
file (e.g. qt_contrib.scm?), because it's a third-party library?

cheers,

Thomas