ggplot: legend: title

This PR is for showing legend **titles** when the legend in not broken for the geom; not for modifying its attributes, like colour.

Summary:
- Retrieve legend title from column name of legend elements in `p$data`; and,
- Create legend title as an annotation, with position `x` and `y` dependent on legend.

Example (to be judged on full-size graph):
Before: https://plot.ly/~pdespouy/2098/posttest-vs-pretest/
After: https://plot.ly/~pdespouy/2099/posttest-vs-pretest/

Image diffs can be seen here: 3c9d75b (PNG rendering is not good, since it should be seen in full-size)

cc/ @mkcor 
I'm not sure of the `y` position.  Since this is hard-coded, can we place this annotation a little higher, with more space below?
I'm looking at https://plot.ly/~TestBot/751 in full-size mode and I find the legend title too close to the actual legend.  `cond` is closer to `A` than `A` is to `B`...  Also, compare to https://github.com/ropensci/plotly/blob/add-r-cookbook-tests/tests/cookbook-test-suite/distributions/box-plot-with-conditions-colored-ggplot2.png
/cc @chriddyp