Improve how file transfer progress is displayed
There are issues with the cumulative nature of the file transfer progress "wheel" in the header bar, while it's somewhat useful as a glance-able element for a single transfer, it kind of falls down as an indicator for multiple transfers:
- it is vague about the progress of individual transfers
- no indication of the number of concurrent transfers
- starting/cancelling additional transfers visually changes the indicator in non-meaningful ways