qmljs: fix race in qmljstodoitemsscanner

avoid calling ui-only method on the ProjectExplorer::Project,
use information cached in ProjectInfo instead.
This (as before) does not automatically scans files in qrc.
Contents of qrc should most likely be added to ProjectInfo.sourceFiles

Task-number: QTCREATORBUG-12556
Change-Id: Ic7f72589cc5bb132ef9d3d03be9d9fc47f1196fd
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>