chore(context-pad): fix selection
chore(util): rename TextUtil -> Text and fix test cases

Related to bpmn-io/bpmn-js#115
feat(core/Canvas): compute absolute bbox based on shape data

This commit adjusts Canvas#getAbsoluteBBox to return the bounding box for
shapes based on the shape data rather than a computed SVG box.

Related to bpmn-io/bpmn-js#115