CellNotes
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Attach notes to cells on a C1FlexGrid.

The sample implements Excel-style cell notes using
two classes:

- CellNote:
CellNote derives from Form and is used to hold, display, and edit notes.
This class is not grid-specific and can be used with other controls.

- CellNoteManager:
CellNoteManager connects to a C1FlexGrid control to provide services
related to the cell notes (mouse handling, owner draw, and a context menu).

