FlexFreezeBottom
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Show frozen rows at the bottom of a grid.

The sample implements a control (derived from Panel) that contains a 
FlexGrid. The contained FlexGrid is positioned with a negative Y 
coordinate within the panel so that the fixed rows are not visible.

The contained grid is also sized and scrolled via code so that the 
bottom 'n' rows are visible.

The contained grid is bound to a main FlexGrid and therefore shares 
the underlying grid model (data, layout, styles, selection, etc).

The whole control (panel and contained grid) is attached to the main 
FlexGrid and positioned at its bottom. This way, the control always
shows the bottom 'n' rows of the main grid.

This is a fairly advanced sample. Most of the code is dedicated to
synchronize the grids and keep an illusion that they are the same
control, even as the user resizes and makes selections.
