chore(properties): rename method Activiti -> Activity

Related to CAM-1605
chore(dependencies): make JDT optional

This commit cleans up the modeler dependencies and makes the JDT (java
development tools) an optional dependency. It is only used for the
"Choose Class" button anyway, which is not shown unless the JDT is
present.

This is a first step towards a standalone modeler.

Closes CAM-1491