Allow configuring FreeMarker (per project)

Some configuration settings affect parsing, and more such settings may appear in the future. A simple yet quite flexible and future-proof way of allowing configuring per project is adding a multi-line text input with label "Configuration settings (Java properites format)" under Project properties / FreeMarker, where the user can enter arbitrary text, then parse that with java.util.Properties and pass it to Configuration.setSettings(Properties).