gtk/window: Make 'default-size' adapt to configured size

gtk/window: Make 'default-size' adapt to configured size

This commit changes the behavior of window size computation and the
default size properties to:

 * The default-width and default-height properties are updated to the
   current window size unless the size is fixed by e.g. being maxmized,
   tiled etc.
 * The compute-size semantics are to just pick the default size, or if
   not adequate, use the measured size, and consequently update the
   default size, unless unresizable.
 * gtk_window_get_size() is removed, what's more likely relevant is the
   gtk_window_get_default_size() which will now contain more sensible
   values.

Various places that used gtk_window_get_size() were updated to use
gtk_window_get_default_size() to remember and restore previous sizes.

This also changes the default value of 'default-width' and
'default-height' from -1 to 0. The gtk builder simplify tool is taught
how to omit when the default size is set to both -1 and 0.