- 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