Enable directory picking with the HTMLInputElement: webkitdirectory property

[Jaifroid]

This might best be done as part of the #656, as the fallback for directory picking. Without the File System Access API, it would only be a very small win, because the user would still have to pick the directory containing ZIM archives every time they open the app. It would only be faster when selecting a different ZIM during the same session.

[Jaifroid]

As mentioned, this is really a sub-issue of #656, and it may be easier to work on it holistically, because it's nearly the same underlying code for the File System Access API, to which supporting webkitdirectory is a kind of footnote (but an important one, because it works in Firefox, in Edge Legacy, and a number of other places where FSA doesn't work). So I'll add some comments in #656.

[Jaifroid]

@Rishabhg71