1. Grid cell defaults are now handled by an internal wxGridCellAttr

1. Grid cell defaults are now handled by an internal wxGridCellAttr
object.

2. wxGridCellRenderer::Draw updated to expect an attr object and use
it instead of going back to the grid for everything

3. The selection colours are now initialized from wxSystemSettings,
and are configurable.

4. Now either CreateGrid() or SetTable() can be called to initialize
the grid.  Added a test for using a non-default table.

5. Reduced flicker even more byt using empty EVT_ERASE_BACKGROUND
handlers.

6. The beginings of wxGridCellEditor are in there but not activated
yet. (This is the next step.)

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@5985 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775