chore(core): improve import performance

This commit improves the import performance of elements by

* directly adding shapes / connections to the elementRegistry instead
running via the eventBus
* initializing event storage only when neccessary
* creating graphics .djs-visuals on demand

Related to bpmn-io/bpmn-js#126
fix(interaction): update connection .djs-hit on change

Closed bpmn-io/bpmn-js#49
fix(core): properly destroy ElementRegistry
fix(draw): use display:none for invisible elements
docs(core): fix minor naming issue
chore(project): update to snapsvg v0.3.0
chore(util): improve text layouting speed

This commit caches the text element to improve the layouting speed.

Related to bpmn-io/bpmn-js#126