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. :^)

WindowServer/WSMenuItem.cpp

@@ -1,9 +1,10 @@
 #include "WSMenuItem.h"
 
-WSMenuItem::WSMenuItem(unsigned identifier, const String& text)
+WSMenuItem::WSMenuItem(unsigned identifier, const String& text, const String& shortcut_text)
     : m_type(Text)
     , m_identifier(identifier)
     , m_text(text)
+    , m_shortcut_text(shortcut_text)
 {
 }