Improvement on catalog product create/edit page

[apiuser]

Feature request from katesuykovskaya, posted on GitHub Mar 26, 2016

Steps to reproduce

1. M 2.0.2
2. go to admin panel -> Products -> Catalog -> edit product
3. open Images and Videos tab, click on image - dialog is shown
4. open source of page

Actual result

1. each time, when modal dialog is created, the old one isn't removed from DOM => when I choose images a lot of time, I got this
   which makes using current / active dialog inconvenient using JS.

Expected Result

1. I should get one dialog, or at least one dialog per chosen image - it should not be duplicated.