Move QGridLayoutEngine to QtGui module

For MacOS 10.6 the exported symbols must be the same regardless of
debug/release configuration, because for debug builds it will by
default link to a release library.

We therefore cannot let some exported symbols depend on if QT_DEBUG
is defined or not. We therefore introduce QGRIDLAYOUTENGINE_DEBUG,
which is independent of debug/release configuration.

The whole dump() function concept should probably be revisited, but
I don't want to clutter this commit too much with such unrelated
things.

Change-Id: I7086f31e2c36fe22ce9c9a3eda37ea25302459a9
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>