Connection labels - #334

Adds optional text labels to connections.

• label field on connections in .pvm (UI-only, ignored by code generation), documented in the pvm spec
• Label is drawn as text on the middle of the longest route segment and highlights with the edge on hover, selection and handle hover
• Double-click an edge or use "Edit Label" in the edge context menu to edit inline; Enter or a click outside commits, Escape cancels, empty text removes the label
• Labels are kept on copy/paste and duplicate; changes are undoable