task pool: orphaned tasks cannot be operated on

[oliver-sanders]

In some situations, tasks which are in the pool, but were orphaned by reload/restart cannot be operated on (e.g, set or removed).

The reproduction for this bug is the same as for #7196

This is a case which was thought about at the time (and I think tested - confirmed), so make sure to check the test when fixing.

[oliver-sanders]

Unfortunately, there are multiple discrete issues here:

• data_store: orphaned n=0 tasks not present in store #7196 is hiding the tasks from the GUI.
• Additionally, orphaned tasks don't exist in inheritance, so cycle / "root" family triggers don't work.
• Even if you operate on the tasks by exact ID, the "on-sequence" check used in ID matching is filtering the tasks out.
• But this is hiding further issues as the remove, set and trigger operations would all fail if the tasks weren't filtered out (KeyError on retrieving TaskDef instance).