fix: trigger external change handling when reloading sessions

[ouuan]

Description

Save the timestamp when saving the session. When restoring a session, compare the timestamp with the file modification time. If the file is newer than the session, trigger an external file change event.

Related Issues / Pull Requests

This should fix #1059.

Motivation and Context

#1059, and this can also prevent accidentally overriding external file changes when CP Editor is not running.

How Has This Been Tested?

On Arch Linux,

1. external file changes when CP Editor is not running
2. kill CP Editor by SIGKILL to simulate abnormal exit

Checklist

• If the key of a setting is changed, the old attribute is updated or it is resolved in SettingsUpdater.
• If there are changes of the text displayed in the UI, they are wrapped in tr() or QCoreApplication::translate().
• If needed, I have opened a pull request or an issue to update the documentation.
• If these changes are notable, they are documented in CHANGELOG.md.