Update Images Changed on Disk

This is for #6356. Continued work @couzteau 's work (pull requests #6443, #7952) to only check that the file was changed on disk and refresh and not close/reopen the file unnecessarily.
It's hard to keep track of `_currentlyViewedPath` and `_currentlyViewedFileMTime` members in this file. Sometimes they are set directly, sometimes they are set in helper functions:
* `_clearCurrentlyViewedPath()`
* `_setCurrentlyViewedPath()`
* `_updateFileModificationTime()`

Besides the initialization at top of module, I think it would be much easier to follow this code if the `_currentlyViewedPath` and `_currentlyViewedFileMTime` members were only ever changed in the helper functions. You could even combine `_clearCurrentlyViewedPath()` and `_setCurrentlyViewedPath()` to simplify it a little more.