Incremental additions to measure tool.

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


In a little while, though, a rework should remove the floating windows entirely, uncluttering the main canvas, and allow the measure tool to stay active during editing, updating and following snapped points.