Fix recommended dictionary download queue

[khaitruong922]

• The download queue does not work because the _importDictionaries call is not awaited so the function ends immediately without checking any other item in queue.
• When the dictionary is in queue, disable its download button. Currently, if the user reopens the recommended dictionary modal, the download button is still active.

[khaitruong922]

Current state: Try to click download other dictionaries while a dictionary is downloading, the buttons are disabled but they are not enqueued.

[Kuuuube]

Any reason for replacing the shift with [0] and moving the shift down later? From a quick look that change appears to be unnecessary.

[khaitruong922]

This is to keep track exactly which dictionary is in download queue.
The user can close the recommended modal and reopen it again. At this point, it will disable download button if the download is in queue. If we remove from the queue before the download is finished then the button will be enabled even though it is being downloaded.