UX: Improved process model labels

Significant improvements made to message flow and sequence flow labels. Labels now remain bound to their position on the connector after you move them Message flow labels are positioned by default closer to the activity or event (as opposed to the blackbox pool) Conditional flow labels (from gateways) are positioned by default on the first segment of the closest to the gateway Undo/Redo no longer breaks label positions in edge case scenarios The position of labels are only reset when you merge segments in a connector