Restore project's open documents

## Problem

Since #912, closing a project folder closes all associated documents. Reopening the project requires the user to reopen desired documents one at a time which is inconvenient.

## Proposal

Projects remember and restore which documents were open when they were closed.

It may be better/easier to store this information in the project folder itself rather creating a separate setting or store (in case the project gets moved) but would need to ensure it is ignored by git.

## Prior Art

Geany does this (optionally using a separate project info folder)