Add PieceTable::apply_change() and an initial undo

src/gui/Window.cc

@@ -270,6 +270,10 @@ void Window::handle_key(uint32_t scancode, uint32_t mod)
     case SDL_SCANCODE_P:
         insertion_test();
         break;
+    case SDL_SCANCODE_U:
+        m_buffer->piece_table->undo();
+        redraw();
+        break;
     }
 }