Fix crash with recursively loading cached compilation units

Fix crash with recursively loading cached compilation units

When a cached compilation unit depends on an import that is implemented as a
Qml C++ plugin and that plugin in turn loads an asynchronous component that is
cached, we would get a crash in the typeloader and a failing assertion
(ASSERT: "d->m_mainThreadWaiting == false" in file qml/ftw/qqmlthread.cpp, line 300)

This is because we did not implement the asynchronous loading within the loader
thread for cached compilation units. This is simply done using a posted event,
using the same mechanism used for other async load methods.

Change-Id: Iefce67ab634ce26122c348dcdfc8e66b00fec671
Reviewed-by: Lars Knoll <lars.knoll@digia.com>