Re-design target addition

[jku]

Fixes #32

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Adding a target only modifies a single metadata so making it a "edit" sub command made sense... However, often user does not know which metadata should be modified: this is especially the case with succinct delegations.

Make add-target a top level action instead of a edit subaction:

• role can still be optionally specified but it is a starting point for the search
• delegation tree is followed from the starting role until the first leaf role (that does not delegate further) is found
• command now also outputs which role it inserted the target file into

This means add-target is now a simple repository wide command which seems quite logical: user does not need to know the details, they just want to add a target file to the repository.

There is a edit()-refactor here that makes the edit-contextmanager handle AbortEdit silently: the users of edit() can raise AbortEdit to signal they want to exit the contextmanager without saving the edited metadata. This is a bit undiscoverable and not immediately obvious but works quite nicely in both snapshot and add-target in my opinion.

[jku]

Closing this and opening a new one from fork (I can't modify this branch anymore since my permissions in this repo were downsized)