Observer classes now has a default value. This simplifies the code in many
   places.

* Improve notification layout

TODO: - get rid of the harcoded path for the icon
      - make the meta printed an option.

     - anchors have a new "lefttop" attribute, allowing them to follow the
       border of the window when resizing
     - the resizing is now handled by the WindowManager class (to enforce the
       mediator pattern). This makes it possible to handle tricky cases,
       such as:
           window B is anchored to window A, and A is resized
           ==> B moves accordingly, and can still be anchored by other windows

* Add a prefs button

 - simple screenshot function
 - an helper function to check if the input item is playing 

- vlcshell.cpp: make opengl the default video output module for macosx
- vlc.r: remove Quicktime mimetype from supported list
- configure.ac: the OpenGL provider requires the AGL framework

    automake naming scheme.

(Remove ITEMPLAY method, which had become redundant)