Quit button, big changes to content system. Moved mde.gui.content to mde.content to reflect it's not only used by the gui. Split Content module into Content and AStringContent. New AContent and EventContent class. Callbacks are now generic and implemented in AContent. Renamed TextContent to StringContent and ValueContent to AStringContent.

--- a/mde/gui/widget/layout.d	Wed Nov 26 13:07:46 2008 +0000
+++ b/mde/gui/widget/layout.d	Sat Nov 29 12:36:39 2008 +0000
@@ -19,7 +19,7 @@
 import mde.gui.widget.Widget;
 import mde.gui.exception;
 
-import mde.gui.content.Content;
+import mde.content.Content;
 
 import tango.util.container.HashMap;