New feature: Background grid (not for IE8)
This change adds a possibility to show background grid as a backgound
image. It's a enabled by default. After this change, IE9 and
Safari/Chrome/Webkit are forced to use pixel sizes instead of fluid
percentages. Optionally, if background is disabled, then IE9 and
Chrome/Webkit can be explicitly told to use percentages with
GantWidget.setAlwaysCalculatePixelWidths(false).