Fix bar chart conversion for position="dodge" (barmode="group" in Plotly)

[mkcor]

Also including fairly unrelated commits in here:

• removing image files from master (they will live on a branch dedicated to the image diff framework);
• style in image-diff--related files (will move to above-mentioned branch);
• updating NEWS file.

This PR offers a working conversion for ggplot2 plots made with geom_bar() and position_dodge(): before (https://plot.ly/~ggplot2-cookbook/16) vs after (https://plot.ly/~marianne2/80). Strengthened related test accordingly.

/cc @pedrodz @chriddyp

[chriddyp]

Nice! where can we check out the image diffs now?

[mkcor]

It's coming, it's coming...
As you can see, a test is failing, my workaround didn't catch this case, where

g$data
  PANEL    fill       x  y fill.name  x.name group ymin ymax xmin xmax
2     1 #F8766D    <NA> 14       Bio  Canada     1    0   14 0.55 1.00
1     1 #00BFC4    <NA> 23      Math  Canada     2    0   23 1.00 1.45
3     1 #F8766D Germany 37       Bio Germany     3    0   37 1.55 2.45
4     1 #00BFC4     USA 20      Math     USA     4    0   20 2.55 3.45

not all x values are NA...
Let me fix this first.

[mkcor]

@chriddyp Image diffs can be seen over here: https://github.com/ropensci/plotly/compare/add-r-cookbook-tests 💃
Ctrl-F for "bar-dodge" at fe2a484

[mkcor]

Before: https://plot.ly/~marianne2/85, after: https://plot.ly/~marianne2/86
Rerunning tests on branch add-r-cookbook-tests now...

[mkcor]

This last commit does not affect 4ca177b
This should be ready for review now :)
Thanks, @chriddyp
We can reorganize/improve the add-r-cookbook-tests branch incrementally, and as we deem appropriate.

[pedrodz]

Tested... Looks great. Thanks @mkcor!