Show a project's own stashes in the prompt stash

[Zheckan]

Summary

Save the current project with each prompt stash, and show stashes per project in the stash menu. Stashes from other projects stay one click away instead of mixed into the list. I think this should be an optional setting.

Problem

I work on several projects at once and stash prompts as ideas for later. The stash is one global list, so when I open it in one project I scroll past ideas meant for others. With an empty composer, Cmd+S restores a single stash directly, so it can even restore another project's prompt into the wrong thread.

Proposal

1. A new stash records the project it was created in.
2. The stash menu lists the current project's stashes. An All projects option shows the rest.
3. Cmd+S with an empty composer considers only the current project's stashes.
4. Stashes saved before this change have no project and appear in every project, so nothing disappears.

I think this should be an optional setting, because some people use the stash as one shared list across projects. With the setting off, the stash works as it does today. New stashes would still record their project, so turning the setting on later sorts them correctly.