File modified on disk not reloaded when XAML Studio opened

If you modify a file on disk outside of XAML Studio, it is not reloaded if that file is opened in XAML Studio when XAML Studio is re-opened.

I believe this is due to the caching/auto-save mechanism in XAML Studio, we're just loading the cached content of the file vs. loading fresh content from disk of the file's state.

**_We should only do that in the case that the file is unmodified, as otherwise we could lose in-progress state._**

A separate feature would be having a file watcher for opened files to determine if they're changed outside of XAML Studio while running and warn the user of such like other applications, should create a separate tracking issue for that...