Make sure .qch file is opened read-only

So far qsqlite tried to open the file first read/write, and falling back
to read access only if this fails. This has resulted in excessively long
loading times e.g. on Windows XP.

Task-number: QTCREATORBUG-10697
Change-Id: I7b1dbddb73feb34491ece5a0765a67354b937f43
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Karsten Heimrich <karsten.heimrich@digia.com>
Reviewed-by: Mark Brand <mabrand@mabrand.nl>