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

include/wx/gtk/textentry.h

@@ -52,6 +52,10 @@ public:
     void SendMaxLenEvent();
 
 protected:
+    // This method must be called from the derived class Create() to connect
+    // the handlers for the clipboard (cut/copy/paste) events.
+    void GTKConnectClipboardSignals(GtkWidget* entry);
+
     virtual void DoSetValue(const wxString& value, int flags);
     virtual wxString DoGetValue() const;