Qt: decode file-browser preview images on a worker thread #3967

Job	Run time
msys2-build-test (CLANGARM64, windows-11-arm)	10m 0s
msys2-build-test (CLANG64, windows-latest)	6m 34s
msys2-build-test (MINGW64, windows-latest)	8m 47s
msys2-build-test (UCRT64, windows-latest)	9m 2s
	34m 23s

Provide feedback