MDL-40648 Added useful functions find_button() and count_button_rows()
MDL-40648 Use new function in parent class, increase sortorder.
MDL-40648 Make sure the button is added in the correct row

previously code expected to find the button in particular row but the tinymce setting may change the buttons' layout
MDL-40648 unittests for editor_tinymce_plugin
MDL-40648 annotated new functions in upgrade.txt