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.

[Satish-Medar]

@Jaifroid I’m happy to work on this issue. Could you please assign it to me?

[Jaifroid]

@Satish-Medar This is quite a complex issue that requires some familiarity with the code base and how ZIM extraction works. I suggest you read carefully CONTRIBUTING, set the project up, play with it a bit, then come back here and explain how you would fix this issue.