TextEditor: Add vim status indicators to the statusbar

When using the VimEditingEngine in the TextEditor, vim's mode and the
previous key are shown in the editor's statusbar.

Userland/Libraries/LibGUI/VimEditingEngine.h

@@ -30,20 +30,48 @@
 
 namespace GUI {
 
+enum VimMode {
+    Normal,
+    Insert,
+    Visual
+};
+
 class VimEditingEngine final : public EditingEngine {
 
 public:
+    VimEditingEngine();
+
     virtual CursorWidth cursor_width() const override;
 
     virtual bool on_key(const KeyEvent& event) override;
 
-private:
-    enum VimMode {
-        Normal,
-        Insert,
-        Visual
+    class PreviousKey {
+    public:
+        PreviousKey() = default;
+        PreviousKey(const KeyEvent& event)
+            : key(event.key())
+            , code_point(event.code_point())
+        {
+        }
+
+        bool operator==(const KeyCode& key) const
+        {
+            return this->key == key;
+        }
+
+        bool operator==(const u32& code_point) const
+        {
+            return this->code_point == code_point;
+        }
+
+        KeyCode key {};
+        u32 code_point {};
     };
 
+    Function<void(VimMode)> on_mode_change;
+    Function<void(const PreviousKey&, bool has_previous_key)> on_previous_keys_change;
+
+private:
     enum YankType {
         Line,
         Selection
@@ -61,7 +89,26 @@ private:
     void update_selection_on_cursor_move();
     void clear_visual_mode_data();
 
-    KeyCode m_previous_key {};
+    // FIXME Support multiple previous keys, this is a temporary measure.
+    PreviousKey m_previous_key {};
+    bool has_previous_key { false };
+
+    void set_previous_key(PreviousKey event)
+    {
+        m_previous_key = event;
+        has_previous_key = true;
+        if (on_previous_keys_change)
+            on_previous_keys_change(m_previous_key, has_previous_key);
+    }
+
+    void clear_previous_key()
+    {
+        m_previous_key = {};
+        has_previous_key = false;
+        if (on_previous_keys_change)
+            on_previous_keys_change(m_previous_key, has_previous_key);
+    }
+
     void switch_to_normal_mode();
     void switch_to_insert_mode();
     void switch_to_visual_mode();