Implements OnMainThreadBlocked.

[lifengl]

This PR is to address #1273.

It is to allow other feature to detect whether a blocking waiting task is actually blocking the main thread. It could allow code to detect certain deadlocks (for example, blocking to wait solution loading completion but the current task is blocking the solution to load.) Or it could raise the priority of the current task in a throttling queue (like design time build queue etc).