To measure tool, add:
        - snap to points, configurable snap distance
        - configurable line and and text colors
        - configurable window forground and background colors
        - lingering measurement, including floating window, cancelled by mouse-down of measure tool
        - measure tool dialog on double-click of measure tool icon
        - optional horizontal and vertical distance indication on canvas
Merge pull request #237 from jhps/measure-tool-linger

Incremental additions to measure tool.