Window: add replace method

permits to replace selection by a string in a single undo step

src/window.cc

@@ -139,6 +139,14 @@ void Window::append_noundo(const String& string)
     scroll_to_keep_cursor_visible_ifn();
 }
 
+void Window::replace(const std::string& string)
+{
+    scoped_undo_group undo_group(m_buffer);
+    erase_noundo();
+    insert_noundo(string);
+}
+
+
 bool Window::undo()
 {
     return m_buffer.undo();

src/window.hh

@@ -56,6 +56,7 @@ public:
     void erase();
     void insert(const String& string);
     void append(const String& string);
+    void replace(const String& string);
 
     const BufferCoord& position() const { return m_position; }
     DisplayCoord   cursor_position() const;