histogram colorization and cleanup

- time_histogram now gets the colors for its bars from the report via the
  colorize() method
- made port colors just a bit lighter
- plot::rgb_t RGB values now default to 0.0
- address histogram now colored by report as set by quick_config()
- cleaned up time_histogram
  - got rid of unused build_legend()
  - choose_subtitle now more appropriately called build_axis_labels
- port histogram now caches derived histogram data so its available to both the
  renderer and to the report without building twice