Improve stash view, support applying stash file/hunk/line path

[sbillig]

Improves the stash view, so that it shows staged, unstaged, and untracked file sections.

Also adds support for applying any portion of a stash (file/hunk/line) as a patch to the current worktree.

Checklist

• The modified code has some test-coverage (where applicable).
• make test is passing (this is what CI runs).
• New unreleased features/fixes/styling and performance improvements are documented via git: feat: / fix: / style: / perf:. Or e.g. perf(highlighting):.
  See https://github.com/altsem/gitu/blob/master/docs/dev-tooling.md