Improved expression editing in Python Calculator

The Python Calculator panel now provides helpers to build expressions without memorizing the syntax.

• Input: selects which connected input to browse arrays from. Disabled when only one input is connected. When Use named inputs is checked, generated accessors use the dataset name (e.g. inputs["Sphere"].PointData["Velocity"]) instead of numeric indexing (e.g. inputs[0].PointData["Velocity"]).
• Array: lists arrays from the selected input, optionally filtered to the current Array Association. Clicking inserts the appropriate accessor at the cursor.
• Function: searchable list of Python functions available in the calculator's namespace, with a one-line summary per entry. Hovering shows the full documentation. Double-clicking inserts function() at the cursor with the cursor placed between the parentheses.

The default Array Name is now Result (capitalized), consistent with the regular Calculator filter. Backwards compatibility has been handled.

vtkPVArrayInformation gains an IsGlobal flag to identify arrays living in root-level composite field data, used by Python Calculator to generate correct accessors for global arrays.

https://discourse.paraview.org/t/improved-python-calculator-expression-editor/17621

This MR resolves #23284.