Eliminate unnecessary indirection in TableSection.
Add cell class support.

This reverts commit 59ad06768182d628c4646e93a245189a75c93a5b.
Fix Cell::setRowspan() and Cell::setColspan() to make sure the object is immutable.
Add CellInterface::hasRange()
Rename CellMatrix::addCell() to CellMatrix::brushAddCell()
Rename CellMatrix::paintShadow() to CellMatrix::brushPaintShadow()
Rename CellMatrixTest::testCellMatrix() to ::testCellMatrixWithBrush().
Fix Cell::getColspan()
Add doc description for CellMatrix::brushPaintShadow()
Add CellMatrix::renderAsTable() for easier testing.
Add brushless API for CellMatrix and use it.
Remove all brushy stuff.