Eclipse designer removes important attributes

When saving a file, the Eclipse designer removes the following attributes from serviceTask nodes:

```
flowable:async="true" flowable:exclusive="false" flowable:class="MyClass"
```

The most important one being `class`. Then, when I run my project, it blows up with something like:

```
org.flowable.engine.common.api.FlowableException: Errors while parsing:
[Validation set: 'flowable-executable-process' | Problem: 'flowable-servicetask-missing-implementation'] : One of the attributes 'class', 'delegateExpression', 'type', 'operation', or 'expression' is mandatory on serviceTask. - [Extra info : processDefinitionId = myProcessDefinition | processDefinitionName = MyProcessDefinition | | id = test | | activityName = Test | ] ( line: 7, column: 5)
```

Original issue: https://github.com/flowable/flowable-engine/issues/738