Save focus level on close

[ergl]

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.

[alok]

If I run this, it seems to clear my current document (good thing I backed up). Other than that it seems to work, and it doesn't clear its own doc.

[ergl]

@alok Sorry to hear that! I should probably test this more. I did this change a long time ago and my document seems to work fine. Let me close it and I'll look at it again

[alok]

@ergl How did you autogenerate data_model.rs with the updated changes? I poked around the protobuf crate and had trouble figuring it out.

[ergl]

I installed the protobuf-codegen plugin for protoc. Once you have that installed, doing protoc --rust_out src/pb/ include/model.proto should do the trick.