Add the possibility to flag svg's as being decorative

[laurent-b4y]

When adding svg's to a site, this is always mainly going to be decorative, in which case having an alt text isn't required, nor a good idea. The svg image module should provide an option to flag the svg as being decorative and thus having the possibility to have a null alt text. This module does it the right way, https://www.drupal.org/project/accessible_media_embed.

Drupal core does this for the image embed in the ckeditor as well, where entering "" double quotes in the alt field output's an empty alt tag.

Now I had to add + 80 alt tags to icons, just to remove them afterwards using js... doesn't make much sense