QmlDesigner.PropertyEditor: extend lookup rules for extensions

If specifics for pane .qml files are located inside the plugin
in the designer sudirectory, we still required the path to be fully
qualified.
e.g. myComponnents/designer/myComponent/MySpecifics.qml

This is unconvinient and also a behaivour change for pure directory imports.
Pure directory imports were not fully qualified until recently.

With this patch we also allow:
myComponnents/designer/MySpecifics.qml

Change-Id: I9a61016f425caf5d7d15140554c72f4a69a6687a
Reviewed-by: Marco Bubke <marco.bubke@digia.com>
Reviewed-by: Eike Ziller <eike.ziller@digia.com>
C++: Fix resolving ui_* files in CppPreprocessor

The working copy contains the artificial ui_* files and therefore we
have to consider it while resolving include files.

Task-number: QTCREATORBUG-9683
Change-Id: Icb3387b4cd885b3652bae3f1da465d3e0f633332
Reviewed-by: Christian Stenger <christian.stenger@digia.com>
Reviewed-by: Eike Ziller <eike.ziller@digia.com>
Reviewed-by: Daniel Teske <daniel.teske@digia.com>
Reviewed-by: Robert Loehning <robert.loehning@digia.com>
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>