silx.gui: support imageaggregate in _plot2d #1728

Job	Run time
Python 3.8 on ubuntu-20.04 PyQt5	10m 20s
Python 3.11 on ubuntu-latest PyQt6	10m 10s
Python 3.12 on ubuntu-latest PySide6	9m 14s
Python 3.10 on macos-13 PyQt5	11m 7s
Python 3.12 on macos-13 PyQt6	12m 16s
Python 3.9 on macos-13 PySide6	11m 58s
Python 3.9 on windows-latest PyQt5	10m 32s
Python 3.12 on windows-latest PyQt6	9m 35s
Python 3.10 on windows-latest PySide6	11m 12s
	1h 36m 24s

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