qt: add action "Copy Selection" to the playlist context menu

We have already been providing text/uri-list when the items are dragged since !5899 (merged), which is useful when the items are dragged to outside the application. In this merge request, I propose doing the same for the global clipboard by providing an option to the user.

I have not assigned Ctrl+C shortcut for this, because it is already assigned to capture device dialog.

Request review @chub.