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.cpp

@@ -0,0 +1,14 @@
+#include <LibGUI/GAction.h>
+#include <LibGUI/GActionGroup.h>
+
+void GActionGroup::add_action(GAction& action)
+{
+    action.set_group({}, this);
+    m_actions.set(&action);
+}
+
+void GActionGroup::remove_action(GAction& action)
+{
+    action.set_group({}, nullptr);
+    m_actions.remove(&action);
+}