Browser: Add "New tab" action (Ctrl+T) :^)

This also introduces a WindowActions collection of actions that are not
specific to the currently open tab, but nevertheless part of its menus.

Applications/Browser/WindowActions.h

@@ -0,0 +1,21 @@
+#pragma once
+
+#include <LibGUI/Action.h>
+
+namespace Browser {
+
+class WindowActions {
+public:
+    static WindowActions& the();
+
+    WindowActions(GUI::Window&);
+
+    Function<void()> on_create_new_tab;
+
+    GUI::Action& create_new_tab_action() { return *m_create_new_tab_action; }
+
+private:
+    RefPtr<GUI::Action> m_create_new_tab_action;
+};
+
+}