Create naming convention for extensions for forms

Currently, Activiti Explorer determines by the availability of a formKey that a form should be displayed. But if users use external form rendering, this doesn't work properly.

The solution is to have a naming convention:

• org/activiti/example/someform.juel.form
• org/activiti/example/someform.ftl.form

Where somename.$

.form

Maybe we should remove the extension from the form key: activiti:formKey="org/activiti/example/someform"

Then Activiti Explorer can calculate if there is a form for that activity: if there is a resource available that matches with one of the names of the build in form engines.