protocol: reorder wl_data_offer.source_actions and wl_data_device.enter

Most compositors send the wl_data_offer.source_actions event before the
wl_data_device.enter event, i.e. after creation of the data offer. This
contradicts to the wayland spec.

On the other hand, it's reasonable to send all the information useful to
the client before the enter event, rather than send mime types before the
enter event and source actions (that don't depend on drop target) after
the enter event.

On the client side, toolkits such as Qt and GTK already expect to see the
source actions before receiving the enter event.

Given all of that, this change adjusts the spec to match the behavior
observed in the compositors in the wild.

Signed-off-by: Vlad Zahorodnii <vlad.zahorodnii@kde.org>
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