HTMLViewTrait: Add preferredWidth/Height properties.
HTMLViewTrait: Add preferredWidth/Height properties.

These are needed by the new StackView for computing how many stacked portions
can be shown at a time.

osric-the-knight/foam,jacksonic/foam,jacksonic/foam,jlhughes/foam,mdittmer/foam,foam-framework/foam,mdittmer/foam,jlhughes/foam,jacksonic/foam,jlhughes/foam,foam-framework/foam,foam-framework/foam,jacksonic/foam,foam-framework/foam,mdittmer/foam,osric-the-knight/foam,foam-framework/foam,osric-the-knight/foam,osric-the-knight/foam,jlhughes/foam,mdittmer/foam