Excel: stop freeze when detecting overflow/cropped text (#6526)
* Excel: stop freeze when detecting overflow/cropped text where an entire row is merged.
Specifically:
* rename _getCellWidthAndTextWidth to _getCellTextWidth and no longer return the cell width.
* In __overlapInfo: No longer use a for loop to calculate the total column width for a merge area, instead just use the first and last column's left and width to calculate it.
* in __overlapInfo: Fetch coordinates and widths as points rather than units, and use Excel's own pointsToScreenPixelsX, which may solve the DPI issue.
