LibGUI: Add a GToolBar class that can be populated with GActions.

The same action can be added to both a menu and a toolbar.
Use this to put a toolbar into FileManager. This is pretty neat. :^)

LibGUI/GToolBar.h

@@ -0,0 +1,25 @@
+#pragma once
+
+#include <LibGUI/GWidget.h>
+
+class GAction;
+
+class GToolBar : public GWidget {
+public:
+    explicit GToolBar(GWidget* parent);
+    virtual ~GToolBar() override;
+
+    void add_action(RetainPtr<GAction>&&);
+    void add_separator();
+
+private:
+    virtual const char* class_name() const override { return "GToolBar"; }
+    virtual void paint_event(GPaintEvent&) override;
+
+    struct Item {
+        enum Type { Invalid, Separator, Action };
+        Type type { Invalid };
+        RetainPtr<GAction> action;
+    };
+    Vector<OwnPtr<Item>> m_items;
+};