DictionaryGrid.editable does not work for CheckColumn

DictionaryGrid.editable setter currently loops over the columns in a grid, setting the editable property on the columns renderer. This is an improvement from the previous behavior, but it doesn't work for CheckColumn because gtk.CellRendererToggle doesn't have an editable property, but instead has an activatable. (Annoying!)

I think the way to fix this is to add a .set_editable on GridColumn to set the editable property on its renderer, and override that in column types whos renderers doesn't support that property.

I'll fix this ASAP.