option in BE for lazy loading images in the gallery content element

# Feature Request
An option for lazy loading images in the new gallery content element.
Right now you can only add a pagination if the number of images exceeds the total number of images per page which can be set in the backend.

## Describe the solution you'd like

A solution would be the new "lazyload"-option which you can be set via an attribute: Currently only supported in Chrome based browsers: https://caniuse.com/#search=lazy

A fall back could be a javascript solution or show a info message which browsers are support. If there is no native browsersupport, fallback to "show all images at once". (If a javascript library as a fallback is used, the library should only be included on the page with the lazyload images.)

The issue names no files, tests, or entry points. Start by locating the gallery content element and its backend configuration, then trace how its images are rendered. Done means the backend exposes a lazy-loading option, the rendered images use it where supported, and the chosen fallback behavior is defined and verified.

Written by the indexing model from the issue text.