LibGUI: Move shortcut actions from GEventLoop to GApplications.

I'm gonna want to have nested event loops sooner or later, so let's not
pollute GEventLoop with things that are meant to work globally.

This patch also changes key events to pass around their modifiers as a
bitfield all the way around the system instead of breaking them up.

LibGUI/GShortcut.h

@@ -7,14 +7,14 @@
 class GShortcut {
 public:
     GShortcut() { }
-    GShortcut(unsigned modifiers, KeyCode key)
+    GShortcut(byte 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; }
+    byte modifiers() const { return m_modifiers; }
     KeyCode key() const { return m_key; }
     String to_string() const;
 
@@ -25,7 +25,7 @@ public:
     }
 
 private:
-    unsigned m_modifiers { 0 };
+    byte m_modifiers { 0 };
     KeyCode m_key { KeyCode::Key_Invalid };
 };