Add TestFrontend documentation and UndoSystem buffer reference update.

- Document `TestFrontend` for programmatic testing, including examples and usage details.
- Add `UpdateBufferReference` to `UndoSystem` to support updating buffer associations.

TestRenderer.cc

@@ -0,0 +1,10 @@
+#include "TestRenderer.h"
+#include "Editor.h"
+
+
+void
+TestRenderer::Draw(Editor &ed)
+{
+	(void) ed;
+	++draw_count_;
+}