Fixes the modal pane to cover the whole screen even if scrolled.  Courtesy of @fairbanksg.  Closes #909.
Reverts removal of default modal pane in SC.PalettePane.  Although it doesn't make sense, it kind of makes sense...
- Removes layout observer from every SC.Pane instance and removes duplicate layout updating on panes.  SC.Pane has support for requesting that the body overflow be auto or hidden depending on the size of the pane and the window.  Only SC.MainPane should ever make use of this though and it can be done using the statechart callback methods instead of an observer.
- Fixes bug that failed to remove the request when a main pane is removed.
- Also simplifies the existing code that disables the body overflow for SC.PickerPanes and actually, you know, makes it work…
Improves layoutDidChange for panes to ensure that the updateLayout code is only run once per run loop.