Resolve "Support toggling check state for all selected items in a tree view"

Closes #18157 (closed)

1. Adds support for toggling checkstate for all selected items in a pqTreeView by simply clicking on one of the items in the selected set.
2. Variable list, block selection widget, and multiblock inspector, all use pqTreeView and hence support ability to collectively change selection state (#18157 (closed)). This also gets rid of the slight UX discrepancies between variable selection widgets and block selection widgets (former we using a pqTreeWidget, instead of pqTreeView).
3. Added tests for the both the select-and-toggle-checkbox functionality and pqHeaderView.
4. Improved appearance of header.

before (linux):

after (linux)

before (macOS)

after (macOS)