CelView: Rename QActions of Add Frame to Append Frame

This patch renames all QActions instances in CelView from Add Frame to Ap
Frame, this way it's more clearer for the user, since "Insert" option is
already adding frames, "Add" right now seems to be too ambiguous.

source/celview.cpp

@@ -394,7 +394,7 @@ void CelView::ShowContextMenu(const QPoint &pos)
     QObject::connect(&action0, SIGNAL(triggered()), mw, SLOT(actionInsertFrame_triggered()));
     contextMenu.addAction(&action0);
 
-    QAction action1("Add Frame", this);
+    QAction action1("Append Frame", this);
     action1.setToolTip("Add new frames at the end");
     QObject::connect(&action1, SIGNAL(triggered()), mw, SLOT(actionAddFrame_triggered()));
     contextMenu.addAction(&action1);