silx.gui: support imageaggregate in _plot2d #1716

Job	Run time
Python 3.8 on ubuntu-20.04 PyQt5	10m 15s
Python 3.11 on ubuntu-latest PyQt6	8m 57s
Python 3.12 on ubuntu-latest PySide6	8m 54s
Python 3.10 on macos-13 PyQt5	14m 4s
Python 3.12 on macos-13 PyQt6	14m 12s
Python 3.9 on macos-13 PySide6	11m 52s
Python 3.9 on windows-latest PyQt5	11m 36s
Python 3.12 on windows-latest PyQt6	10m 50s
Python 3.10 on windows-latest PySide6	9m 1s
	1h 39m 41s

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