split __repr__ into draw and __repr__ and set interactive=False in draw()

`draw()` only draws the plot without showing it, `__repr__()` then uses
plt.show() to open the plot (which only does it if interactive == True
and a suitable backend is used). This change is needed for saving a
plot via `ggsave(plot)`.

To make this possible, interactive = True has to be set to False
while in `draw()`. Set interactive=True for the rest
(= `__repr__()`) during importing and based on `sys.flags.interactive`.
Add a fullfeatured ggsave command

Mostly it is feature complete to the one from ggplot2, including
filenames, sizes and making it possible to save a specific plot.
TST: Don't test images other than png

The test default to test png, pdf and svg images, but this
a) doesn't work on stock windows and
b) is probably irrelevant as we don't want to test backends.

It also was already disabled for some tests.

So disable every extension but png.
TST: Remove now unneeded baseline images (pdf, svg)
TST: tool to show all visual tests in the browser
TST: Update scale_reverse test und baseline images

The test images are still not perfect (-> legends) but the main functionality
is working and looking good in the tests.