Freeform "attribute" editor

This implements aeva#70 (closed) by allowing users to create, modify, and remove simple properties on SMTK components.

These properties are denoted "freeform attributes" in the user interface; "attribute" is used since they are analogous to smtk::attribute::Attribute (but much simpler) and "freeform" is used to distinguish them from structured, validated Attribute values.