Add element templates utilities to API

[smbea]

This PR adds the removeTemplate, unlinkTemplate and updateTemplate utilities to the element templates API.

This will make https://github.com/camunda/camunda-bpmn-js/issues/250 easier

[nikku]

Let's document them here as a follow-up.

[smbea]

I implemented your suggestions. Two things:

• the newTemplate is null in applyTemplate, it will unlink not remove - so I added that event
• I noticed applyTemplate does the exact same thing that unlinkTemplate does if newTemplate=null. We could just remove unlinkTemplate but I decided to keep unlinkTemplate with a call to applyTemplate(element, null) to avoid having to add null in each call. But feel free to disagree here

[barmac]

Happy to see this merged 🎉