Update Viewer for PyQt6, improve load time

[jsiirola]

Fixes # .

Summary/Motivation:

The implementation of the Model Viewer's extension point for the pyomo command caused Qt (and all the dependent libraries) to be imported when the pyomo command loaded the pyomo.command extension points (taking nearly 14 seconds!). This PR reworks the extension point so that Qt (et al.) are not imported when initializing the pyomo script environment.

This PR also adds support for using PyQt6 (in addition to PySide6 and PyQt5).

Changes proposed in this PR:

• Rework how the model Viewer is registered with the Pyomo command (split the QtApp out from the command registration)
• Add support for PyQt6

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