silx.gui: support imageaggregate in _plot2d #1684

Job	Run time
Python 3.8 on ubuntu-20.04 PyQt5	10m 22s
Python 3.11 on ubuntu-latest PyQt6	8m 58s
Python 3.12 on ubuntu-latest PySide6	7m 5s
Python 3.10 on macos-13 PyQt5	16m 57s
Python 3.12 on macos-13 PyQt6	14m 38s
Python 3.9 on macos-13 PySide6	11m 7s
Python 3.9 on windows-latest PyQt5	8m 44s
Python 3.12 on windows-latest PyQt6	10m 21s
Python 3.10 on windows-latest PySide6	5m 10s
	1h 33m 22s

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