display panel slider using reactor

[cmeyer]

• Minor code change.
• Add raster-set-display-limits action and use it in Histogram.
• Use reactor style tracking for display panel slider.
• Use reactor for image canvas item hand tool.
• Integrate action into hand tool. First pass.
• Add create line mouse handler. Preliminary.
• Continue with line mouse handler, partially generalized.
• Continue more with line mouse handler.
• Use mouse handlers for all graphics in raster image display.
• Use mouse handler for raster pointer tool.
• Remove old mouse tracking code in raster image display.