silx.gui: support imageaggregate in _plot2d #1686

Job	Run time
Python 3.8 on ubuntu-20.04 PyQt5	10m 13s
Python 3.11 on ubuntu-latest PyQt6	8m 38s
Python 3.12 on ubuntu-latest PySide6	6m 46s
Python 3.10 on macos-13 PyQt5	15m 13s
Python 3.12 on macos-13 PyQt6	13m 39s
Python 3.9 on macos-13 PySide6	9m 50s
Python 3.9 on windows-latest PyQt5	8m 58s
Python 3.12 on windows-latest PyQt6	9m 26s
Python 3.10 on windows-latest PySide6	3m 50s
	1h 26m 33s

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