New feature: Implement PickFolderMultiple (#135) #573

Job	Run time
Windows latest - Clang, Static	1m 37s
MacOS latest - Clang, Static	40s
Windows latest - MinGW, Static	1m 26s
MacOS 11 - Clang, Static	35s
Ubuntu latest - GCC, GTK, NoAppendExtn, Static, C++20	31s
MacOS latest - Clang, Shared	37s
Windows latest - MSVC, Static	1m 20s
Ubuntu latest - GCC, GTK, NoAppendExtn, Static, C++11	35s
Windows latest - MSVC, Shared	58s
Ubuntu latest - Clang, GTK, NoAppendExtn, Static, C++20	1m 14s
ClangFormat check	7s
Ubuntu latest - Clang, GTK, NoAppendExtn, Static, C++11	37s
Line endings check	21s
Ubuntu latest - GCC, Portal, NoAppendExtn, Static, C++20	1m 1s
Ubuntu latest - GCC, Portal, NoAppendExtn, Static, C++11	1m 0s
Ubuntu latest - Clang, Portal, NoAppendExtn, Static, C++20	25s
Ubuntu latest - Clang, Portal, NoAppendExtn, Static, C++11	30s
Ubuntu 20.04 - GCC, GTK, NoAppendExtn, Static, C++20	30s
Ubuntu 20.04 - GCC, GTK, NoAppendExtn, Static, C++11	27s
Ubuntu 20.04 - Clang, GTK, NoAppendExtn, Static, C++20	1m 5s
Ubuntu 20.04 - Clang, GTK, NoAppendExtn, Static, C++11	1m 11s
Ubuntu 20.04 - GCC, Portal, NoAppendExtn, Static, C++20	1m 23s
Ubuntu 20.04 - GCC, Portal, NoAppendExtn, Static, C++11	1m 0s
Ubuntu 20.04 - Clang, Portal, NoAppendExtn, Static, C++20	31s
Ubuntu 20.04 - Clang, Portal, NoAppendExtn, Static, C++11	1m 17s
Ubuntu latest - GCC, Portal, AutoAppendExtn, Static, C++11	1m 5s
Ubuntu latest - Clang, Portal, AutoAppendExtn, Static, C++11	41s
Ubuntu latest - GCC, Portal, NoAppendExtn, Shared, C++11	25s
	23m 9s

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