backend: process actions in regard to their priority
    
    The general idea is to put an integer value to `JobQueue.add_task`'s
    `priority` paramenter, then everything works as expected.
    
    For actions this could be straightforward and we could just propagate
    the priority value that we got from frontend directly to the `JobQueue`
    but because this solution should be reusable for builds and potentially
    other separate queues where even backend plays a factor when calculating
    the priority value, we decided to address those needs here.
    
    This implementation provides `QueueTask` which is able to compute the
    final priority based on preferences of both frontend and backend.
    `ActionQueueTask` and all future tasks (e.g. `BuildQueueTask`) should
    inherit it and override either `frontend_priority`,
    `backend_priority` properties or both.