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