LibGUI: Make undo work for TextDocument ReplaceAllTextCommand

The undo code here was just replacing the new text with the new text.
Now we actually save the old text and use that instead. :^)

Userland/Libraries/LibGUI/TextDocument.cpp

@@ -970,7 +970,8 @@ DeprecatedString InsertLineCommand::action_text() const
 
 ReplaceAllTextCommand::ReplaceAllTextCommand(GUI::TextDocument& document, DeprecatedString const& text, GUI::TextRange const& range, DeprecatedString const& action_text)
     : TextDocumentUndoCommand(document)
-    , m_text(text)
+    , m_original_text(document.text())
+    , m_new_text(text)
     , m_range(range)
     , m_action_text(action_text)
 {
@@ -980,7 +981,7 @@ void ReplaceAllTextCommand::redo()
 {
     m_document.remove(m_range);
     m_document.set_all_cursors(m_range.start());
-    auto new_cursor = m_document.insert_at(m_range.start(), m_text, m_client);
+    auto new_cursor = m_document.insert_at(m_range.start(), m_new_text, m_client);
     m_range.set_end(new_cursor);
     m_document.set_all_cursors(new_cursor);
 }
@@ -989,7 +990,7 @@ void ReplaceAllTextCommand::undo()
 {
     m_document.remove(m_range);
     m_document.set_all_cursors(m_range.start());
-    auto new_cursor = m_document.insert_at(m_range.start(), m_text);
+    auto new_cursor = m_document.insert_at(m_range.start(), m_original_text, m_client);
     m_range.set_end(new_cursor);
     m_document.set_all_cursors(new_cursor);
 }

Userland/Libraries/LibGUI/TextDocument.h

@@ -270,17 +270,18 @@ private:
 class ReplaceAllTextCommand final : public GUI::TextDocumentUndoCommand {
 
 public:
-    ReplaceAllTextCommand(GUI::TextDocument& document, DeprecatedString const& text, GUI::TextRange const& range, DeprecatedString const& action_text);
+    ReplaceAllTextCommand(GUI::TextDocument& document, DeprecatedString const& new_text, GUI::TextRange const& range, DeprecatedString const& action_text);
     virtual ~ReplaceAllTextCommand() = default;
     void redo() override;
     void undo() override;
     bool merge_with(GUI::Command const&) override;
     DeprecatedString action_text() const override;
-    DeprecatedString const& text() const { return m_text; }
+    DeprecatedString const& text() const { return m_new_text; }
     TextRange const& range() const { return m_range; }
 
 private:
-    DeprecatedString m_text;
+    DeprecatedString m_original_text;
+    DeprecatedString m_new_text;
     GUI::TextRange m_range;
     DeprecatedString m_action_text;
 };