Planner tasks from notes

Planner tasks from notes should update the notes page that they link
to while maintaining the special links. This means that
planner-jump-to-linked-task needs to be smarter.

Let us look at case 1: Link to plan page note

The plan page has a date link, no anchor. The date page has a plan
link with an anchor. Example:

 #A1  _ Foo (2004.03.11)
 #A1  _ Foo ([[SomePage#1][SomePage]])

Problem! The plan page does not contain enough information to
reconstruct the link if necessary. I can’t think of a neat way to fix
this yet. Can anyone?