New KGraphicsSignalPlotter class

[John Tapsell]

Hi all,

  I want to introduce a new KGraphicsSignalPlotter class, which is a
QGraphicsWidget version of KSignalPlotter.

  Both the KGraphicsSignalPlotter and KSIgnalPlotter are built from
the same cpp file, with a few #ifdef's in the (fairly few) places that
they need to be different.

  The current situation is that there is a copy of an old
KSIgnalPlotter that someone copied to kdelibs/plasma/widgets and
modified.  The code bases are thus diverging, and I really want to
avoid this.  I have put a lot of work into KSIgnalPlotter to make it
faster and better, and it would be nice for the plasma widget to
benefit from this.  I want to reduce this plasma widget to a bare
shell that just inherits KGraphicsSignalPlotter and adds on the few
plasma specifics bits (using theme colors mostly.  Maybe also the SVG
code)

  I tried upload the code to reviewboard, but I can't get it to accept
git's diff format. :-/

John