QmlDesigner.PropertyEditor: Using :ICore::mainWindow() for dialogs

Since the current focus is on (native) QtQuickView window we have to
specify the Qt Creator main window.

Task-number: QTCREATORBUG-11322
Change-Id: I0e05efaa88b6177004730cd8c43010568cf64eac
Reviewed-by: Robert Loehning <robert.loehning@digia.com>