Update image while sliding Z/T (cache planes)

[will-moore]

This has long been requested by many users (and recently requested at https://forum.image.sc/t/a-couple-of-queries-about-omero-web-iviewer-and-search-by-key-value-pairs/47334), but it seems we've not created an issue for it yet.

An investigation PR was started at #316 but ran into issues of cached planes and ROIs not being perfectly in-sync when changing Z/T.
Also, to give the user an idea of what is loaded (e.g. like YouTube or OMERO.player) will need quite a bit of UI work, as will other issues like managing the size of the cache and figuring out the best way to store the planes.

[imagesc-bot]

This issue has been mentioned on Image.sc Forum. There might be relevant details there:

https://forum.image.sc/t/a-couple-of-queries-about-omero-web-iviewer-and-search-by-key-value-pairs/47334/2