ENH: Logic to set correct size of checkbox for QCheckableHeaderView

This change fixes issue with size of checkbox on the Macintosh platform where
the section header is of a much smaller height.

Change-Id: I7a1260f39514167567a8b947c2ce1367ea309329
Merge topic '14580-top-level-checkbox' into next

2a3fcf5c ENH: Logic to set correct size of checkbox for QCheckableHeaderView