Redo Editor textDir support and TogglePlugin to work together, fixes #14950.

Also fixed a bug where Editor toolbar buttons were being
created with weird names like "dijit/editor/_plugins/ToggleTextDir".
On IE that was causing Editor to think that the <input> was the plugin.

Finally, cleaned up the tests to remove code duplication.
The tests could still be much better, for example checking the button state
(checked or not) and checking the computed direction of the editor text rather
than directly checking the dir attribute.