add standard cut/copy/paste actions (if available) to the cpp editor's context menu

at a later stage we will look into harmonizing context menu among different editors.
The cpp editor had the biggest need, because its context menu features all sorts of
extra refactoring operations, thus the standard Qt context menu became too large.

src/plugins/texteditor/basetexteditor.h

@@ -321,6 +321,8 @@ public:
     void setMimeType(const QString &mt);
 
 
+    void appendStandardContextMenuActions(QMenu *menu);
+
     // Works only in conjunction with a syntax highlighter that puts
     // parentheses into text block user data
     void setParenthesesMatchingEnabled(bool b);