grid_columns should have an editable property

It should be possible to easily make columns editable. This would solve many problems.

* https://bugs.launchpad.net/bugs/812266 would be easily fixed by simply looping over the columns and setting editable to the new value without having to call _refresh_treeview() -- which itself causes problems.

* In many instances, it does not make sense to make the entire grid either editable or non-editable. You might want to update a database record, for instance, but that would require you not to edit the record id. Things that must not be changed should not be changable. Being able to do DictionaryGrid.get_column(0).editable = True (or .set_editable(True)) would be a large improvement.