add Window::center_selection method

src/window.cc

@@ -35,6 +35,12 @@ Window::~Window()
     m_option_manager.unregister_watcher(*this);
 }
 
+void Window::center_selection()
+{
+    BufferIterator cursor = selections().back().last();
+    m_position.line = std::max(0, cursor.line() - m_dimensions.line/2);
+}
+
 void Window::update_display_buffer()
 {
     scroll_to_keep_cursor_visible_ifn();

src/window.hh

@@ -30,6 +30,7 @@ public:
 
     const DisplayBuffer& display_buffer() const { return m_display_buffer; }
 
+    void center_selection();
     void update_display_buffer();
 
     String status_line() const;