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.

TestInputHandler.cc

@@ -0,0 +1,38 @@
+#include "TestInputHandler.h"
+
+
+bool
+TestInputHandler::Poll(MappedInput &out)
+{
+	if (queue_.empty())
+		return false;
+	out = queue_.front();
+	queue_.pop();
+	return true;
+}
+
+
+void
+TestInputHandler::QueueCommand(CommandId id, const std::string &arg, int count)
+{
+	MappedInput mi;
+	mi.hasCommand = true;
+	mi.id         = id;
+	mi.arg        = arg;
+	mi.count      = count;
+	queue_.push(mi);
+}
+
+
+void
+TestInputHandler::QueueText(const std::string &text)
+{
+	for (char ch: text) {
+		MappedInput mi;
+		mi.hasCommand = true;
+		mi.id         = CommandId::InsertText;
+		mi.arg        = std::string(1, ch);
+		mi.count      = 0;
+		queue_.push(mi);
+	}
+}