wayland: Send wl_data_offer.source_actions before wl_data_device.enter

This makes kwin's behavior consistent with other wayland compositors
(sway, GNOME Shell, etc) and it's reasonable to provide all the
information about the data source before wl_data_device.enter. It also
makes Firefox happier.

Relevant discussion upstream: https://gitlab.freedesktop.org/wayland/wayland/-/issues/322

CCBUG: 445661


(cherry picked from commit 3546cc05)