LibGUI: Add Widget focus proxies

A Widget can now have a focus proxy widget. Questions about focus are
redirected to the proxy if present. This is useful if a widget could
logically get focus, but wants one of its child widgets to actually
handle it.

Libraries/LibGUI/Widget.cpp

@@ -499,8 +499,22 @@ void Widget::set_window(Window* window)
     m_window = window;
 }
 
+void Widget::set_focus_proxy(Widget* proxy)
+{
+    if (m_focus_proxy == proxy)
+        return;
+
+    if (proxy)
+        m_focus_proxy = proxy->make_weak_ptr();
+    else
+        m_focus_proxy = nullptr;
+}
+
 bool Widget::is_focused() const
 {
+    if (m_focus_proxy)
+        return m_focus_proxy->is_focused();
+
     auto* win = window();
     if (!win)
         return false;
@@ -514,6 +528,9 @@ bool Widget::is_focused() const
 
 void Widget::set_focus(bool focus, FocusSource source)
 {
+    if (m_focus_proxy)
+        return m_focus_proxy->set_focus(focus, source);
+
     auto* win = window();
     if (!win)
         return;

Libraries/LibGUI/Widget.h

@@ -150,6 +150,10 @@ public:
     bool is_focused() const;
     void set_focus(bool, FocusSource = FocusSource::Programmatic);
 
+    Widget* focus_proxy() { return m_focus_proxy; }
+    const Widget* focus_proxy() const { return m_focus_proxy; }
+    void set_focus_proxy(Widget*);
+
     enum class ShouldRespectGreediness { No = 0,
         Yes };
     struct HitTestResult {
@@ -340,6 +344,8 @@ private:
     bool m_accepts_emoji_input { false };
 
     NonnullRefPtr<Gfx::PaletteImpl> m_palette;
+
+    WeakPtr<Widget> m_focus_proxy;
 };
 
 inline Widget* Widget::parent_widget()