Generate clipboard events for wxComboBox in wxGTK too.

These events were only generated for wxTextCtrl but should be sent for
non-readonly wxComboBox too, so refactor the code to allow its reuse from
wxComboBox.

Also add EVT_TEXT_PASTE handlers for both controls to the widgets sample for
testing.

Closes #14520.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72252 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/gtk/combobox.cpp

@@ -60,6 +60,7 @@ gtkcombobox_popupshown_callback(GObject *WXUNUSED(gobject),
     event.SetEventObject( combo );
     combo->HandleWindowEvent( event );
 }
+
 }
 
 //-----------------------------------------------------------------------------
@@ -167,6 +168,8 @@ bool wxComboBox::Create( wxWindow *parent, wxWindowID id, const wxString& value,
 
         g_signal_connect_after (entry, "changed",
                                 G_CALLBACK (gtkcombobox_text_changed_callback), this);
+
+        GTKConnectClipboardSignals(GTK_WIDGET(entry));
     }
 
     g_signal_connect_after (m_widget, "changed",