fix(core/EventBus): correctly indicate ok on #fire quick return
chore(core/Canvas): provide a default layer
chore(styles): remove overlay clutter
feat(context-pad): allow custom actions

This commit enables custom actions (mousedown / dragstart) to be handled
in context pad providers.

Related to bpmn-io/bpmn-js#107
feat(connect): add visual connection of elements

This commit adds the capability to connect elements visually via the
context pad. It provides a new service `connect` that takes care of the
connecting elements.

It extends the modeling service with two methods that handle connecting
elements. Implementors must implement

* modeling#canConnect(source, target): return true if connecting
elements is possible
* modeling#connect(source, target): connect elements

Related to bpmn-io/bpmn-js#107