Added 'Open with ->Qt Designer' in Project Explorer.

Added IExternalEditor which knows a kind and a mimetype.
Make EditorManager and ProjectExplorer "Open With" query
the interface and add the respective kinds.
Add "openExternalEditor" to EditorManager.
Add External editors for Designer and Linguist,
making use of Mac 'open' or Designer's Tcp socket mechanism
to ensure files are opened in the same instance (per Qt version).

Task-number: 249392
Reviewed-by: con <qtc-committer@nokia.com>

src/plugins/qt4projectmanager/qt4projectmanagerplugin.cpp

@@ -42,6 +42,7 @@
 #include "profilereader.h"
 #include "qtversionmanager.h"
 #include "qtoptionspage.h"
+#include "externaleditors.h"
 
 #include <coreplugin/icore.h>
 #include <extensionsystem/pluginmanager.h>
@@ -133,6 +134,13 @@ bool Qt4ProjectManagerPlugin::initialize(const QStringList &arguments, QString *
     addAutoReleasedObject(new Qt4RunConfigurationFactory);
     addAutoReleasedObject(new Qt4RunConfigurationFactoryUser);
 
+#ifdef Q_OS_MAC
+    addAutoReleasedObject(new MacDesignerExternalEditor);
+#else
+    addAutoReleasedObject(new DesignerExternalEditor);
+#endif
+    addAutoReleasedObject(new LinguistExternalEditor);
+
     // TODO reenable
     //m_embeddedPropertiesPage = new EmbeddedPropertiesPage;
     //addObject(m_embeddedPropertiesPage);