Spreadsheet view poorly supported mutli-component arrays. This commit
adds support to Spreadsheet view to show mutli-component arrays under a
single header-section which spans all the components. This enables us to
resize the component-section individually, and yet easily realize that
the sections correspond to the same array.

For this we add a new pqMultiColumnHeaderView that combines adjacent
sections with the same `DisplayRole` text.

vtkSpreadSheetRepresentation now splits multi-component arrays so each
component shows up separately. We also add magnitude array
(fixes #18431).

This cleanup header section resize to fix #18430.

Fixes #16126.