TINY-631: disable button and menu item, when there are no appropriate headers for the toc; address some edge cases
TINY-631: disable button and menu item, when there are no appropriate headers for the toc; address some edge cases

blakewallan/tinymce,blakewallan/tinymce,tinymce/tinymce,ikuchmin/tinymce,danielpunkass/tinymce,ETSI-CTI/TinySpecMCE,tinymce/tinymce,gencer/tinymce,MediehusetTek/tinymce,leofeyer/tinymce,rbecheras/tinymce,TeamupCom/tinymce,rbecheras/tinymce,aduth/tinymce,ETSI-CTI/TinySpecMCE,ikuchmin/tinymce,aduth/tinymce,fillabs/TinySpecMCE,FernCreek/tinymce,jtark/tinymce,FernCreek/tinymce,danielpunkass/tinymce,jtark/tinymce,leofeyer/tinymce,adaptiffdesigns/tinymce,fillabs/TinySpecMCE,adaptiffdesigns/tinymce,gencer/tinymce,TeamupCom/tinymce,MediehusetTek/tinymce,FernCreek/tinymce,danielpunkass/tinymce,tinymce/tinymce