Display aggregate installation progress in dock icon

[interstateone]

At a minimum, Xcodes should display an appropriate overlay on the dock icon when one or more versions are being installed. This could be modelled as a boolean internally.

A fancier implementation of this could provide richer indication of the aggregate installation progress, perhaps through a tree of Progress objects for all versions that are being installed, and each step within their installation.

A library that might make this easier to implement is https://github.com/sindresorhus/DockProgress