Closing a project window waited on the event-loop thread for the project's
language servers, kernels and debug sessions to shut down on the async
runtime. When the runtime workers were busy answering IPC or querying
windows, they waited for the event loop in turn and the app froze.
Window destruction and the close_project command now detach the project at
once and let its workers shut down in the background. Opening a project
waits for those shutdowns first, and exit waits for them with a bound.
Closes#1414
l0ng-ai
0d485e67d5