Set system property for security profile

[hallvard]

A new version of PlantUML is recently published which introduce the notion of "Security Profile"
See https://plantuml.com/en/security

This is quite useless for the Eclipse Plugin, but this is necessary for web server hosting.
In some future, the default security level will be changed to INTERNET which may cause issues for the Eclipse Plugin because it will disable reading data from local file.

The solution is quite simple (you can even implement it now):
System.setProperty("PLANTUML_SECURITY_PROFILE", "UNSECURE");
However, you might allow users to change the security profile used by the Eclipse Plugin through some UI. Personally, I think that it makes no sense and that UNSECURE should be the default for such a plugin.