Suggest existing Fields when creating new ones (similar to tags)

[ognsya]

This would help make sure the use of Fields is consistent across a Workspace.

Note this wouldn't work for the case where a user first writes the field name and then transforms the node into a field. However, to be honest I think that workflow is a bit awkward. Like tags, I always know when I'm about to create a Field. So I'd rather have a trigger for that (for example , double ":"), which would create the Field on ENTER (like when creating a tag), and move the cursor to the value.