wayland: Fix drag-and-drop issues in Firefox
At the moment, the data offer initializes supported and preferred action to "none". But if the target doesn't accept anything the source provides, the compositor may still need to send action events with "none"
This change makes data offer and data source provide dnd actions wrapped in std::optional in order to indicate if they have been set explicitly.
Aleix was right!
Draft: not sure about xwayland changes
Edited by Vlad Zahorodnii