Populate Qt Assistant's TOC from documentation directory.

Task-number: QTBUG-37643
Change-Id: I8c17dc02e8d6c17c88dde24454f09dba76f32148
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>
Reviewed-by: Karsten Heimrich <karsten.heimrich@digia.com>