LibGUI: Add MoveLineUpOrDownCommand

This allows lines moved by Ctrl+Shift+[Up, Down] to be registered as a
command, i.e. cancellable by Ctrl+Z.

This patch also introduces the usage of TextDocument::[take,
insert]_line. Those functions forward changes to the visual lines and
then avoid some data mismatch.

Co-authored-by: Jorropo <jorropo.pgm@gmail.com>

Userland/Libraries/LibGUI/TextEditor.h

@@ -229,6 +229,12 @@ public:
 
     virtual Optional<UISize> calculated_min_size() const override;
 
+    template<class T, class... Args>
+    inline void execute(Badge<EditingEngine>, Args&&... args)
+    {
+        execute<T>(forward<Args>(args)...);
+    }
+
 protected:
     explicit TextEditor(Type = Type::MultiLine);