tdf#120625: All vcl windows now share 1 instance of clipboard

so we can copy'n'pasta between apps (or 2 instances of the same app)
Copy'n'pasta into system clipboard is still buggy for non-Writer apps

Change-Id: I07f025172654dd3e6c0ef9cd5ad78d24224f039f
Reviewed-on: https://gerrit.libreoffice.org/64721
Tested-by: Jenkins
Reviewed-by: Katarina Behrens <Katarina.Behrens@cib.de>
(cherry picked from commit 1e3d6c80)
Reviewed-on: https://gerrit.libreoffice.org/65153Reviewed-by: Michael Weghorn <m.weghorn@posteo.de>

vcl/inc/qt5/Qt5Instance.hxx

@@ -40,6 +40,7 @@ class VCLPLUG_QT5_PUBLIC Qt5Instance : public QObject,
     osl::Condition m_aWaitingYieldCond;
     int m_postUserEventId;
     const bool m_bUseCairo;
+    css::uno::Reference<css::uno::XInterface> m_xClipboard;
 
 public:
     std::unique_ptr<QApplication> m_pQApplication;

vcl/qt5/Qt5Instance.cxx

@@ -244,10 +244,14 @@ Qt5Instance::CreateClipboard(const css::uno::Sequence<css::uno::Any>& arguments)
                                                   css::uno::Reference<css::uno::XInterface>(), -1);
     }
 
-    css::uno::Reference<css::uno::XInterface> xClipboard(
-        static_cast<cppu::OWeakObject*>(new VclQt5Clipboard()));
+    if (!m_xClipboard.is())
+    {
+        css::uno::Reference<css::uno::XInterface> xClipboard(
+            static_cast<cppu::OWeakObject*>(new VclQt5Clipboard()));
+        m_xClipboard = xClipboard;
+    }
 
-    return xClipboard;
+    return m_xClipboard;
 }
 
 Reference<XInterface> Qt5Instance::CreateDragSource()