LibGUI+WindowServer: Add app-global keyboard shortcuts.

This patch adds a GShortcut class. Each GAction can have a GShortcut which
will cause the event loop to listen for that key combination app-globally
and activate the event in case it's pressed.

The shortcut will also be displayed when the action is added to a menu.

Use this to hook up Alt+Up with the "open parent directory" action in the
FileManager app. :^)

LibGUI/GShortcut.h

@@ -0,0 +1,42 @@
+#pragma once
+
+#include <Kernel/KeyCode.h>
+#include <AK/AKString.h>
+#include <AK/Traits.h>
+
+class GShortcut {
+public:
+    GShortcut() { }
+    GShortcut(unsigned modifiers, KeyCode key)
+        : m_modifiers(modifiers)
+        , m_key(key)
+    {
+    }
+
+    bool is_valid() const { return m_key != KeyCode::Key_Invalid; }
+    unsigned modifiers() const { return m_modifiers; }
+    KeyCode key() const { return m_key; }
+    String to_string() const;
+
+    bool operator==(const GShortcut& other) const
+    {
+        return m_modifiers == other.m_modifiers
+            && m_key == other.m_key;
+    }
+
+private:
+    unsigned m_modifiers { 0 };
+    KeyCode m_key { KeyCode::Key_Invalid };
+};
+
+namespace AK {
+
+template<>
+struct Traits<GShortcut> {
+    static unsigned hash(const GShortcut& shortcut)
+    {
+        return pair_int_hash(shortcut.modifiers(), shortcut.key());
+    }
+};
+
+}