Allow tasks to have backlinks

Currently, if I link a task anywhere, it gets added to the context. If I remove objects from the context section, they 'disappear' since they can't be seen in backlinks.

And a bug related to this: when objects in a task's context are removed b clicking x, on the next reboot, they come back as if they were never removed.

Main reason for not wanting to have all links as context is that it can become really cluttered esp for recurring tasks if one has a journaling practice that will include mentioning said task.