Added live-refresh to FolderListingPureWidget.

Improved const correctness and parameter names in Operations.cpp.
FolderListingWidget no longer resets the entire selected folder, but rather just the selection within child folder (which is more direct alongside live refreshing, and avoids a visible flash).

It's no longer necesssary to force reset the entire selected folder
thanks to live refreshing of folder listings.
Changed `MenuWidget<std::string> *` usages to more appropriate
`FolderListingPureWidget *`.