LibGUI: Add GActionGroup, a way to group a bunch of GActions.

This can be used to make a bunch of actions mutually exclusive.
This patch only implements the exclusivity behavior for buttons.

Libraries/LibGUI/GActionGroup.h

@@ -0,0 +1,35 @@
+#pragma once
+
+#include <AK/HashTable.h>
+#include <AK/Weakable.h>
+
+class GAction;
+
+class GActionGroup : public Weakable<GActionGroup> {
+public:
+    GActionGroup() {}
+    ~GActionGroup() {}
+
+    void add_action(GAction&);
+    void remove_action(GAction&);
+
+    bool is_exclusive() const { return m_exclusive; }
+    void set_exclusive(bool exclusive) { m_exclusive = exclusive; }
+
+    bool is_unchecking_allowed() const { return m_unchecking_allowed; }
+    void set_unchecking_allowed(bool unchecking_allowed) { m_unchecking_allowed = unchecking_allowed; }
+
+    template<typename C>
+    void for_each_action(C callback)
+    {
+        for (auto& it : m_actions) {
+            if (callback(*it) == IterationDecision::Break)
+                break;
+        }
+    }
+
+private:
+    HashTable<GAction*> m_actions;
+    bool m_exclusive { false };
+    bool m_unchecking_allowed { false };
+};