Vsevolod Kukol

Moved API change from #8338 to this new PR. * WelcomePage API now async, like WelcomeWindow * All async members return Task

[Core][Tests] Add basic ProgressMonitor tests

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This PR adds more tests for progress monitors. Some tests are based on assumptions that are not necessarily right and need clarification. See: #7256.

Bring back instant Ide theme switching without the need to restart the Ide. > Fixes VSTS [676682](https://devdiv.visualstudio.com/DevDiv/_workitems/edit/676682)

[Ide][Mac] Allow to exit by closing the main IDE window

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I don't know why, but we don't exit if the user closes the main IDE window on Mac. Instead we just hide it (we event don't minimize with the typical...