API inconsistencies

[schani]

• onVisibleRowsChanged is badly named. It affects columns, too. Maybe onVisibleRegionChanged? - yes
• onColumnMoved takes a column index, but onColumnResized takes a GridColumn. - that's fine for now
• We use an array for X/Y coordinates, a Rectangle object for rectangles, and drawCustomCell takes x/y/width/height as four separate arguments. - make drawCustomCell take a Rectangle
• EditableGridCell should have a common newValue property #5 - make the underlying data always present as data, with displayData being there sometimes
• allowInsertRow and onRowInserted should probably be called "add" or "append" instead of "insert" - showTrailingBlankRow and onRowAppended
• getCellsForSelection is passed a GridSelection, but only its rectangle is needed - fix it