Fix view menu toggle labels not updating after hide (#381)

## Summary

- Implements `splitView:canCollapseSubview:` (returns YES) so
NSSplitView tracks collapsed state
- Adds `editorVisible` / `previewVisible` properties to MPDocument based
on subview width
- Updates `validateMenuItem:` to toggle "Hide/Restore" labels using
visibility state
- Removes a guard in `setDividerLocation:` that skipped `setPosition:`
at ratio 0/1, which prevented NSSplitView from registering collapsed
subviews

Related to #377

## Test plan

- [x] 849 tests pass (0 unexpected failures)
- [x] View → Hide Editor Pane → label changes to "Restore Editor Pane"
- [x] View → Restore Editor Pane → editor restores, label changes back
- [x] Same for preview pane

Author: schuyler