Fix drag and drop with touch between different windows

When dragging from one window to another, we may end up in a data_device that didn't get "data_device_start_drag". In that case, the internal touch point serial will be incorrect and we need to update it to the serial from the seat. The serial stored in the seat is changed to std::optional so we can properly check if it is set.

Draft because this requires changes in Qt, otherwise source clients will crash when finishing a drag.