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.
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