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.cpp

@@ -0,0 +1,26 @@
+#include "WindowActions.h"
+#include <LibGUI/Window.h>
+
+namespace Browser {
+
+static WindowActions* s_the;
+
+WindowActions& WindowActions::the()
+{
+    ASSERT(s_the);
+    return *s_the;
+}
+
+WindowActions::WindowActions(GUI::Window& window)
+{
+    ASSERT(!s_the);
+    s_the = this;
+    m_create_new_tab_action = GUI::Action::create(
+        "New tab", { Mod_Ctrl, Key_T }, [this](auto&) {
+            if (on_create_new_tab)
+                on_create_new_tab();
+        },
+        &window);
+}
+
+}