Constructs a Notes Sheet from the given Notes record.
Initialises TextRuns, to provide easier access to the text
super(notes, notes.getNotesAtom().getSlideID());
// Now, build up TextRuns from pairs of TextHeaderAtom and
// one of TextBytesAtom or TextCharsAtom, found inside
// EscherTextboxWrapper's in the PPDrawing
_runs = findTextRuns(getPPDrawing());
// Set the sheet on each TextRun
for (int i = 0; i < _runs.length; i++)
_runs[i].setSheet(this);