[CDF-287] - As a user, I want to be able to define a chart made up of arbitrary, compatible, plots.

* A few file renames of files that weren't following the normal naming.
* Refactored the legend renderer classes.
* Changed the structure of legend scenes; now not following the "one group per color axis and data cell" rule; now, one color axis has one legend group; within the same legend group, multiple data cells may be present and each corresponding plot panel may register a legend symbol renderer; multiple visible renderers are rendered overlapped.
* Visibility of the legend of a data cell is controlled by the corresponding visual role's "legend.visible" option.
* Legend symbol extension points can now be unambiguously specified in the color visual role's legend option object. This substitutes the existing "legend" + index, which are hard to know the indexes of; compatibility was maintained;
* Locking of legend items containing only trend datums, when in visibility clickMode, is now handled by the LegendItemSceneVisibility mixin.
* Updated unit tests relating to visual role's legend options.

* Main site examples changes evidencing new features and ways of doing (plots, tooltip, formats)
* Grouped chart properties by component, making examples easier to understand and compare, and evidencing and transitioning to using plots and visualRoles collections.

* Other relevant changes
  * Added the ibits method to pvc.Interactive so that implementations may lazily calculate it; this was required to support the ability to not hide the last visible legend item.
  * New pvc.finished helper function to make it easier to specify "finished" extension points.
  * Fixed pie plot multi-line linked labels being too close.
  * Fixed pie plot not being able to do multiple plots because of the category and angle axes being shared. The fix is not optimal. Ideally we would want to be able to specify that an own axis (not shared, index auto/ assigned from negative values?) should be created.
  * Fix labels derived by the translator from the column name not being capitalized and split into words.
  * Removed legacy/stale "hidden" def on the legend panel. Added the ability to hide individual items through extension point. No layout reflow though between sections.

Missing changes to JsDocs related to the new visual roles' legend options.

@pamval please review.