Introduce UUID identifier for tasks

[dbarashev]

We need UUIDs to decrease the probability of conflicts in the real-time collaboration, e.g. when two users concurrently create a new task. We will distinguish tasks by UUIDs when sending them to the server and back, however, we'll keep the current ID as the task identifier in the desktop GanttProject. We will allow for the ID duplicates at merge time and will renumber tasks during merging.