MDL-44083 Atto: Adding images for accessibility tools
MDL-44083 Atto: Setting new icons for accessibility tools
MDL-44038 Atto: Store references to toolbar nodes at creation
MDL-44038 atto_: Set button focus after dialogue closure
MDL-44487 Atto: Restrict size of image preview in popup.

Instead of only restricting the width, we set a max-width and max-height in css.
We also make the dialog a bit wider so it looks better in clean.
MDL-44480 Javascript: Correct dialogue ID generation
MDL-43883 Behat: Make Atto the default text editor and adjust behat to use it.
MDL-43883 Atto behat: Rubrics are expecting magic tweaking in text from tinymce.
MDL-43883 Atto Behat: Remove deprecated step
MDL-43883 Atto behat: Make the set_field js handle atto or simple textarea