TestCase: add a function to test for existence of pixels of a certain colour

There's no simple way to check if a grabbed image actually has non-white (or any other colour) pixels. I'm proposing something like this for the type returned by TestCase's grabImage function:

int numberOfPixelsOfColor(color)

or just:

bool hasPixelsOfColor(color)

Our own tests already have similar checks and/or helpers:

• getPaintedPixels in tests/auto/gui/painting/qpainter/tst_qpainter.cpp
• The "foundPixel" checks in in tests/auto/gui/painting/qpainter/tst_qpainter.cpp
• numberOfNonWhitePixels in tests/auto/quick/qquicktext/tst_qquicktext.cpp and tests/auto/quick/qquicktextedit/tst_qquicktextedit.cpp
• containsSomethingOtherThanWhite in tests/auto/quick/scenegraph/tst_scenegraph.cpp

So we should add a function to QTest as well, or even QImage.