ENH: Paint checkbox in QHeaderView

Added new class that derives from QHeaderView and paints a checkbox on the
section header if the individual model items are checkable.
Currently the class is used in pqTreeView and needs to be standardized across
all views and widgets.
Removed old unused pqCheckableHeaderView and pqCheckableHeaderModel classes.
This fixes issue 14580.

Change-Id: Ib9f45794340823819eb3f59dba23461dead53f79
Merge topic '14580-top-level-checkbox' into next

9347b60f ENH: Paint checkbox in QHeaderView