Restarting of downloads paused due to termination of kiwix-desktop

[veloman-yunkan]

On functional side, now when closing kiwix-desktop with a ongoing download, at restart the download is paused. Would be nice to have it automatically restarted (especially as a downloading zim is not visible in local library, the default view).
But can be made in another PR, this one is big enough.

Agree. There are also a couple of edge cases like the book appearing in an obscure state when the app exits while the download request has been enqueued but not yet submitted to aria2 (this is easily cleared via the "Delete book" action but may be confusing to the user). I think that those too better be fixed in a separate PR.

Originally posted by @veloman-yunkan in #1118 (comment)

[veloman-yunkan]

An alternative to automatic resuming of downloads is to let the user know that there are paused downloads and offer them to resume them.