Only kick off project pref loading when the project changes

This is a fix for #7250. What was happening is that there was a file tree refresh occurring and that was causing the `PreferencesManager._setCurrentEditingFile` in `_loadProject` to fire. The purpose of that call is to start the loading of project-level prefs, but in this case we weren't actually loading a new project and the result was that the preferences context for editing was being set incorrectly.