Add progress extension API

[felixfbecker]

Ref #947

[codecov-io]

Codecov Report

Merging #1375 into master will decrease coverage by 0.08%.
The diff coverage is 8.57%.

Impacted Files	Coverage Δ
shared/src/extensions/controller.ts	0% <0%> (ø)	⬆️
shared/src/notifications/Notifications.tsx	0% <0%> (ø)	⬆️
shared/src/notifications/NotificationItem.tsx	0% <0%> (ø)	⬆️
shared/src/notifications/notification.ts	0% <0%> (ø)	⬆️
shared/src/api/client/services/notifications.ts	100% <100%> (ø)	⬆️
shared/src/api/client/connection.ts	84.78% <20%> (-8.08%)	⬇️
shared/src/api/client/api/windows.ts	57.69% <20.83%> (-32.31%)	⬇️
shared/src/api/extension/api/windows.ts	59.57% <5%> (-40.43%)	⬇️

[sqs]

At a high level, this looks exactly right. 👍

[felixfbecker]

Ready for review