Handle changes in the content of a remembered folder (or removal of previously picked archive)

[Jaifroid]

@Rishabhg71 This relates to your File System Access PR. I just realized that we don't handle cases where the folder content has changed. It silently fails if a user clicks on an archive that's no longer in the folder. Also, naive users might not be aware that if they've added an archive to a folder, the list in the dropdown list of archives won't show the new content until they re-pick the archive.

I don't think this should hold up the release of v3.11.0, because it's not critical, and the FSA PR is a huge improvement for end users. So this can be added with a subsequent PR. It could involve a small refresh icon underneath the selected list of archives, and of course throwing up a message to the user if they click on an archive that is not found. It should also handle refreshing the contents of the dropdown list of archives as soon as the user gives permission to access the archive again in a new session, so that the list is as up-to-date as it is possible to be (we can never update it till the user gives permission of course).

[Jaifroid]

For now I'm going to add a message in the UI instructing users to re-pick the content if it has changed.

[Jaifroid]

Additionally, when a user picks a new directory using webkitdirectory API, the old contents are not cleared from the list, and the new contents are, confusingly, added to the dropdown list of archives.

[Jaifroid]

There is some janky code introducing a race condition as commented in #1185 (comment). This could be the cause of some issues with the display of folder contents. Needs to be checked out.