TINY-961: fix button a11y
TINY-961: fix button a11y

tinymce/tinymce,leofeyer/tinymce,leofeyer/tinymce,FernCreek/tinymce,gencer/tinymce,adaptiffdesigns/tinymce,FernCreek/tinymce,adaptiffdesigns/tinymce,danielpunkass/tinymce,TeamupCom/tinymce,danielpunkass/tinymce,danielpunkass/tinymce,MediehusetTek/tinymce,tinymce/tinymce,FernCreek/tinymce,TeamupCom/tinymce,MediehusetTek/tinymce,tinymce/tinymce,gencer/tinymce