Move the dark canvas into PaletteGraphicsView and test it directly

The inverted painting, the rubber band replay and the changed()
receiver lived in DiagramView, which the unit tests cannot link, so the
update-flag regression was only covered through a stand-in view. They
now live in PaletteGraphicsView, a QGraphicsView subclass with no
other dependency, and DiagramView derives from it. The view tells a
subclass through paintingInverted(bool) when it renders for an
inverted display; DiagramView forwards that to the diagram. The grid
dot rule moves out of Diagram::drawBackground into
QET::Palette::gridDotColor().

tst_qetpalette now links the real class: gridDotColorSoftensInvertedDots,
paletteViewFollowsThePalette (light sheet, dark sheet at text contrast
with a red box still red and the paintingInverted calls in order, back
to light), paletteViewKeepsSceneUpdatesFlowing (three whole-scene
updates and a selection each repaint, scene set after construction),
paletteViewDrawsTheRubberBand.

cmake/qet_compilation_vars.cmake

@@ -248,6 +248,8 @@ set(QET_SRC_FILES
   ${QET_DIR}/sources/qet.h
   ${QET_DIR}/sources/qeticons.cpp
   ${QET_DIR}/sources/qeticons.h
+  ${QET_DIR}/sources/palettegraphicsview.cpp
+  ${QET_DIR}/sources/palettegraphicsview.h
   ${QET_DIR}/sources/qetpalette.cpp
   ${QET_DIR}/sources/qetpalette.h
   ${QET_DIR}/sources/qetstyle.cpp