Keep track of selection focus after close

[spacekookie]

From the PR (#18) initially implementing this:

I often use deep trees, and currently every time you reopen void, the view gets reset to the home level. This makes it so the view stack and the current focus view are restored between runs. Maybe it's not the best way to things (just exposing fields with pub), but it might be helpful anyway.
The changes should work on existing documents, as all the new proto fields are optional, but I haven't done exhaustive testing of this.

An alternative implementation was added with PR #27. Unfortunately adding this feature isn't quite that simple. It requires an update to the protobuf generated code which leads to horrible data loss if not done correctly.

Therefore this feature is blocked on a few changes around configurable data formats

• Existing data files (legacy) need to keep working without data loss
• New storage format needs to include current selection
• There needs to be a convenient way to migrate from the legacy format to the new one

I'll also add a milestone to this once we can estimate how much work is actually required for this