feature(outline, selection): allow selection of controls

Limitations: context pad is not functional for connections

See bpmn-io/bpmn-js#121
style(diagram-js.css): new outline style
refactor(outline): extract getConnectionBBox to util
feature(overlay): set correct positioning for overlays attached to connections

See bpmn-io/bpmn-js#121
WIP(styles): use crisp outline