MDL-44126 Atto: List of minor fixes from review.

toolbar order (base it on tinymce)
change plain to paragraph
default for collapse plugin should be 5 groups
fix alignment of table dialogue form
insert newlines in html generated by table plugin
rename accessibility helper to screenreader helper
remove accessibility hint for images and media files
fix some broken plugin names
style table captions
remove verbose accessibility hints
MDL-44127 Atto: Improve Atto API to allow plugins with multiple buttons/menus