This broke the direct inelastic system tests and will need to be
re-worked before being committed again.
This reverts commit c2e67aff.

Re #2656.

Back-ported in from today's QtiPlot.

Seems slightly frivolous, but it was easy so why not.
Involved bringing in the ImageSymbol class from today's QtiPlot and
exposing to python. A custom symbol can be loaded from an image file
or drawn using the QPixmap class.

Some VTK macros (vtkNotUsed) seem to be giving doxygen a lot fits when
trying to location function signatures. As actual VTK/ParaView code doesn't
use doxygen, I'm turning our plugins off to avoid issues as the few ways I've
tried to quiet warning never made doxygen happy.

and add a test helper to the test build instead of a separate target

It doesn't call the Q_OBJECT macro, so moc does nothing but emit
a warning. Even though the class does inherit from QObject, it
fails to compile if I add the macro so I'll follow what the Qt
people do (since this is one of their examples).