QmlDesigner.SubComponentManager: removing bottleneck

The function checkIfDerivedFromItem()is really slow, since it does require
a link. Since not much harm is done when we display QtObjects, I remove the
check for now, until we find a faster solution.

Change-Id: Ibd75bd7ed17cfc8268ec0745e330d75cb4e69ebe
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@digia.com>
QmlDesigner: Get imports from the code model

Change-Id: I96928025aede350bf4ff084555f612568d9389d8
Reviewed-by: Fawzi Mohamed <fawzi.mohamed@digia.com>
QmlDesigner.ItemLibrary: Allow higher version

We should propably introduce a version range.

Change-Id: I74db2ff6efafa26902c5f2a67040661ca6755508
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@digia.com>
QmlDesigner.FormEditor: crash fix

Fix a crash when closing Qt Creator and clear() is called from the
destructor.

Change-Id: I037599653a6ca32e326b2263e7e6f526523e6f8e
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@digia.com>
Doc: update information about Qbs

Fix the link to the newly published Qbs Manual in
qt-project.org/doc/qbs/. Describe how to use the
Qbs project wizards. Update screenshot to show a
project generated by the wizard.

Change-Id: I5fa7ce03499a4fdcdfdeea676a933ded64200598
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>