Manually requested downloads at same time do not queue

[rat-moonshine]

While testing multiple JDK download requests (v11, v8) at a same time, the interface didn't shown the downloads are in queued manner and have started parallelly causing UI notification conflicts (causing by different download events) and some internal problems as well.

We need to fix that the downloads always runs in a queued manner.

[rat-moonshine]

@JoelProminic this is now ready for a test. Please, check.

[JoelProminic]

I tested with the branch build, and queuing is working properly now. I tested on both macOS and Windows. This should be ready for a merge.

[piotrzarzycki21]

I have removed tag. When it will be possible merge it.

[JoelProminic]

These changes were made in master. This is working for the original test cases, but I noticed some potential problems for #32. I'll reopen this issue if I confirm that there is a remaining problem.