Add value modification notification to TabularEditor

When editing an array with the TabularEditor, there is currently no way to get notified when a value is changed, since one can't listen to change inside a NumPy array.

It would be nice to build infrastructure in the `TabularEditor` to simplify this task, for example by allowing to pass an attribute that will be notified by the editor when a value is changed (similarly to the `refresh/update` attributes to request a repaint of the editor).

One way to work around this limitation currently is to keep a handle on the `ArrayAdapter`, subclass it, and add an event notification at the end of its `set_text` method.

cc @corranwebster