Allow selecting file(s) and directorie(s) with the file dialog

[reduf]

This seems to unfortunately not be possible using Windows API (that rfd uses).

One option is to create our own file dialog, but that's quite a bit of extra work. There is a project already doing this work, ImGuiFileDialog but it doesn't integrate so nicely with the OS.

Maybe SHBrowseForFolderW can also be used.