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)

该 issue 未指定任何文件、测试或入口点。首先跟踪项目如何关闭和重新打开文档，然后确定可以在哪里存储项目本地状态，同时使其继续被 git 忽略；当重新打开项目能够恢复之前打开的文档时，即视为完成。

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