Make popover retrieval functions work with libzim

[Jaifroid]

As our javascript-libzim port is getting faster and more useable, at least on desktop, we need to ensure going forward that all added features are agnostic about which back end they use. Popovers currently only work with the standard backend, not with the experimental libzim worker. Since we can get directory entries and contents from either backend, it should not be difficult to abstract the logic for retrieving the popover contents.