MDL-44067 Atto: Improve table editing controls.

Remove flaky inline menu icons - instead clicking on the table button, when
you are in a table shows the context menu for the current cell.

I removed all the special code for 'atto_control' - because it did not work very well. Having non-contenteditable
nodes inside a contenteditable region seems in expose bugs in different browsers.
MDL-44092 Atto: Add selectionchanged event and listeners in the buttons.

This means the buttons can indicate the state of the text under the cursor.