silx.gui: support imageaggregate in _plot2d #1679

Job	Run time
Python 3.8 on ubuntu-20.04 PyQt5	10m 8s
Python 3.11 on ubuntu-latest PyQt6	8m 51s
Python 3.12 on ubuntu-latest PySide6	6m 59s
Python 3.10 on macos-13 PyQt5	13m 14s
Python 3.12 on macos-13 PyQt6	16m 19s
Python 3.9 on macos-13 PySide6	9m 35s
Python 3.9 on windows-latest PyQt5	9m 57s
Python 3.12 on windows-latest PyQt6	9m 49s
Python 3.10 on windows-latest PySide6	4m 45s
	1h 29m 37s

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