Improved layout support

Currently, Reahl deals with layout in several different ways: special widgets (such as LabelledBlockInput), special widgets for use as main window (TwoColumnPage), and general inclusion and use of Yui 2 Grids/Css.

It would be more flexible to implement a kind of LayoutGrid which can be used as a child of a widget and that allows a programmer to do a grid-based layout programmatically. How this will be implemented may relate to our use of Yui 2 Grids CSS - we also need to move on to Yui 3's CSS framework, so looking at this as part of the same job makes sense as well.

(See also: https://groups.google.com/forum/#!topic/reahl-discuss/s4lVeiBG3hI )