Move dataset picker setup into ScatterplotPlugin #786

Job	Run time
prepare_matrix	3s
Cross platform build (Macos_xcode16, macos-15, apple-clang, 16, Release, Macos, 16.2, libc++, armv8)	6m 10s
Cross platform build (Windows-msvc2022, windows-2022, msvc-2022, 17, MD, Release)	6m 41s
Cross platform build (Linux_gcc13, ubuntu-24.04, gcc, g++, gcc, 13, Release, Linux, libstdc++11, ...	5m 7s
	18m 1s

Provide feedback