Improve the style and behavior of the image-viewer element

In order to create a more modern look & feel, the image viewer needs a style more clean, without its dark retro background.
If possible, no bgcolor is required.
This way, it should assume the nuxeo-card background color and having invisible limits.

Other usability details should be considered, as the possibility to pan only the image if it has content bigger than the viewer area.

• Small images should be presented in 100% instead of the full width/height of the viewer. If we present small images with a lot of zoom, they will not perform nicely.

• Images with high resolution should fit to width/height of the viewer, by default, to give the user the possibility to have an immediate overview.

• If possible, insert an icon (see attachments) between the zoom out and the zoom in, that gives the possibility to the user to achieve the real resolution (100%) with only 1 click - and go back to full width of the viewer with only 1 click as well.