Fix "Detach templates" not working when export target is the same file

[Hijanhv]

Summary

Fixes #4456 — when Detach template instances and Repeat last export on save are both enabled and the export target is set to the same file as the map itself, templates were not being detached on subsequent saves.

Root cause (two interacting bugs):

1. ExportHelper::prepareExportMap always clears exportFileName/exportFormat from the exported clone — so the file on disk loses the export settings. On the next reload triggered by the file watcher, those settings are gone, causing exportDocument to early-exit with an empty export filename.

2. The export write updates the file's on-disk timestamp, but mLastSaved is only updated during the regular save. The file watcher therefore treats the export write as an external change and auto-reloads the document, which replaces the in-memory map (including its export settings) with the freshly-loaded one that has no export settings.

Fix:

• In ExportHelper::prepareExportMap, preserve exportFileName and exportFormat in the exported file when the export target and the map file resolve to the same canonical path. This restores persistence across reloads and Tiled restarts.
• Add Document::refreshLastSaved(fileName) and call it after a successful export to the same file (in exportDocument, exportMapAs, and exportTilesetAs). This prevents the file watcher from treating the export write as an external change and triggering an unnecessary auto-reload.

Test plan

• Enable Detach template instances and Repeat last export on save in preferences
• Open a map, do File → Export As… targeting the same .tmx file
• Add template instances to the map, then press Ctrl+S
• Open the saved file in a text editor — template objects should be fully detached (no template= attribute)
• Press Ctrl+S again — templates should continue to be detached on each save
• Close and reopen Tiled, reload the file — export-on-save should still work (export settings persisted in file)
• Verify that exporting to a different file still clears export settings from the exported file (no regression)