Various internal details about the order and error handling of completion
actions are documented in "Concurrent Programming on Windows" by Joe Duffy.

An instance can only have one completion of each type, trying to add a
second one leads to an exception on Windows.

The instance is marked as long-running even if TpCallbackMayRunLong fails,
a second call will lead to an exception on Windows.

user32/tests: Add some tests to show that WS_CLIPCHILDREN doesn't exclude children from update region.

Also lock/unlock the module only on creation/destruction of the object.