Plugin manager/install window is modal, but shouldn't be

[psobolewskiPhD]

I noticed that the plugin install dialog is modal, so if you do get a notification -- e.g. connection issue -- you can't click over to the napari window to read/check it.
Obviously #69 could address the part about not being able to read a notification, but I see no reason the dialog should be modal?
Given that you can install multiple things in batches, having it open but on the side while doing something in napari seems like a totally normal use case.
I've vote that unless there is a compelling reason to make it modal, it shouldn't be modal.
See, e.g. the napari Prefences dialog.