Cache QML import directories and qmldir files.

Cache QML import directories and qmldir files.

Also a little less toUtf8().

Change-Id: Iefb255fad9e27553f29b184418e66ce317f1cd89
Reviewed-on: http://codereview.qt.nokia.com/3748
Reviewed-by: Roberto Raggi <roberto.raggi@nokia.com>
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>