MDL-44216 Atto: Reversing the indent outdent buttons
MDL-44227 Atto: the screenreader helper now displays some texts when alt or title don\'t exist
MDL-44228 add Atto sub-plugins admin table
MDL-44228 editor_atto: added missing outdent icon
MDL-44221 editor_atto: fixed bug creating table in IE <= 10

This involved introducing a insert_html_at_focus_point method for
the editor and using that instead of insertHTML
MDL-44221 editor_atto: convert plugins to use insert_html_at_focus_point
MDL-44218 editor_atto: prevent atto getting massive and allow the user to resize it
MDL-44040 editor: implemented shortcuts for bold, italic, underline and undo
MDL-44223 Atto: Setting images to display a place holder when src is broken
MDL-43864 editor_atto: added a prevent autolink button to atto
MDL-44226 editor_atto: refactored the accessibility checker to make it usable by users who don't know html