Allow queueing downloads

[Efreak]

Currently if you try to download a game while one is downloading, it fails with an error that only one Legendary instance can run at a time. Since it clearly knows when a download is complete (there's a notification), please add a basic queue.

Better yet if such a queue allowed cancelling the current download after leaving the game page.

[mrsjunkrunner]

This is not a simple thing to implement and like most other quality of life features it's on the list.