QmlDesigner: Add a way to find out if instance has a value

It was not possible to find out if a property has value in the node
instance.

Change-Id: I54bf4cddbef431104153555b9c89379a3048cc9c
Reviewed-by: Tim Jenssen <tim.jenssen@digia.com>
QmlDesigner: Fix SubComponentManager::importPaths()

Change-Id: I950c9e2df44d0bf51b9a8b08528116a1ef50823e
Reviewed-by: Tim Jenssen <tim.jenssen@digia.com>
QmlDesigner: Fix import path

The code same code cluttered around and sometimes fixed and sometimes not.

Change-Id: I45d1a7bc85bac14ccfcc3e242321f1260d4293c2
Reviewed-by: Tim Jenssen <tim.jenssen@digia.com>
QmlDesigner: Fix many implementations of getSnapshot

Change-Id: I759956f8090ef9a94d04e6f11da9bbba246e48ac
Reviewed-by: Tim Jenssen <tim.jenssen@digia.com>