refactor: reduce tree specific complexity in commands.py

[toofar]

Several commands related to manipulating tabs now have duplicate logic paths that have to be maintained. For contributors who want to change, or add, commands it would be great if they didn't have to think about tree traversal. It would be great if we could push logic down into the tabbed browser or even the notree data structure.

There are some places where this might not be possible, or some places where we think there is existing non-tree related code that should get the same treatment.

Some ideas:

• for places where we are handling new commands or arguments (like --sibling) this isn't so bad because it isn't a new logic path just to handle tabs, but even then there might be stuff that we could move out to reduce the line noise.
• for recursive operations could we teach the tree structure to take care of them somehow? We may also have to teach the tree about tab type nodes in this case so it can apply actions to them when closed, moved, hidden, whatever (that might even be able to be handled by connected to node signals when adding tabs)
• treating collapsed tabs and hidden nodes specially is a recurring theme that seems error prone. We might want to look at calling code and see if we can teach the tree to take care of all that somehow
• it might not help move things out of the file but it may help to reduce the line count to experiment with the visitor pattern. A lot of cases where the tree is traversed it's only with the intention of taking an action for each node, have a common pattern to take care of more traversal details could help reduce the line count and introduce some more consistency to tree traveral