Allow changing panorama image on hotspot hover

[tf]

On twitter, there has been the suggestion to allow changing the panorama background image when a hotspot is hovered. That way gallery-like effects could be created, where initially only parts of multiple images are visible and individual images can be "expanded" by hovering a hotspot.

Some things to consider:

• While everything that can be achieved with the current form of hover images (which are only displayed inside the hotspot area) could also be done with the suggested technique, the current approach requires downloading only two images instead of one per hotspot. Therefore (and also to preserve backward compatibility) this should be an additional option that can be used side by side with the current hover image. Hotspots with custom background images would not display the configured hover image of the page.

• On touch devices, the hover image is currently displayed by tapping a hotspot. The hotspot stays active until the user either taps another hotspot or the panorama background. If the background image changes when selecting a hotspot, only part of the image would be visible on small viewports. The current behavior would allow looking around. To prevent hiding the alternative background image by accident, one could consider keeping the alternative background until another hotspot is selected.