Rework ObWindow for better inheritence.

Rework ObWindow for better inheritence.

add window_new() that allocates an ObWindow (also works for a subclass)
add window_free() that frees it.
add window_set_abstract() that a subclass must call to set up the ObWindow's
  "abstract class" functionality
add abstract properties in ObWindow: depth, top, layer.
make all the existing ObWindows use the new window_foo() functions.