[FEATURE] Install packages in parallel or in FIFO order in the background.

[Sayamame-beans]

Is your feature request related to a problem? Please describe.
When I want to install multiple packages (e.g., when creating a new project), it is frustrating and tedious to press the add button one by one, wait for the addition to finish, add the next package, and repeat...
I would like to press the buttons consecutively at the beginning and then just respond to the dialog.

Describe the solution you'd like
I would like to be able to press the add button while adding another package and have it placed in the processing queue. Adding in parallel would also be nice, but I believe this could be difficult due to Unity limitations.

Describe alternatives you've considered
Like #187 , it would also be nice to select multiple packages and add them all at once.

Additional context
N/A