When selecting multiple tasks in the task browser, it is possible to add subtasks but not add a parent

[nekohayo]

I just realized that the sensitivity state of these top two menu items doesn't make sense when you select multiple tasks that have no parent:

I would expect being able to add a parent for multiple tasks at once (which, in the current state of things, would mean reparenting if some of those tasks already have a parent; unless eventually we can figure out #634 ...), but not be able to add a subtask to multiple selected tasks (unless again, issue no. 634...).

I'm guessing that if someone uses the "add a subtask" action in this context right now, it will either break or it will add a subtask only to the task that was under the mouse cursor when right-clicking (or maybe the first task in the list of selected tasks).

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I would be tempted to consider this an easy ("low-hanging-fruit") issue, but I suspect it will be heavily dependent on #553 to land first.