Opening the same image in multiple viewers results in detections being wrongly shown in both

[petebankhead]

Bug report

Describe the bug
There's a problem with the tile caching v0.3.2 and v0.4.2 (probably other versions) when using the the same image opened in multiple viewers.

Basically, detections made in one image can show up in the other. The reason is that tiles that show the detections are being cached using the same key in both viewers, because the underlying image files are the same.

To Reproduce

1. Create a project & add an image
2. Duplicate the image in the project
3. Open the original image & its duplicate in different QuPath viewers
4. Detect cells in a small region in one viewer
5. Detect cells in a different region in the second viewer
6. See the cells magically appear in both viewers

Expected behavior
It's possible to work with the same image in multiple viewers.

Screenshots

Desktop (please complete the following information):

• OS: All
• QuPath Version: v0.4.2 (also v0.3.2 and likely other versions)

Additional context
Because it's a tile caching issue, the problem isn't evident when zoomed in (since the tile cache isn't used to paint detections).

The issue was first reported at https://forum.image.sc/t/qupath-problem-with-showing-detections-in-multi-view/76334/3

The cell counts etc. should be unaffected.

One workaround is to duplicate the underlying image file, so that it has a different key for caching (based upon the file name/path).

[petebankhead]

The fix seems straightforward, just to remove the getPath() method from
https://github.com/qupath/qupath/blob/v0.4.2/qupath-gui-fx/src/main/java/qupath/lib/gui/images/servers/PathHierarchyImageServer.java#L151

I'll submit a PR, the main question is whether there will be a v0.4.3 or if the next release will be v0.5.0 (further in the future).