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

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