FIX: Check for x/y == None in facet_wrap and new tests

If you only supply one variable (x or y) to facet_wrap, the other
would be "None", resulting in a `df.get(None)` call, which doesn't
anymore work in newer pandas. So add a few checks for None.

Also change the __radd__() method, so that only the copy of the ggplot
object is changed. This also needs adjustments in the tests.

Decouple visual tests and tests of internals in test_scale_facet_wrap

Add a new test that __radd__() does not change the original object.
FIX: Reduce the max image size in ggsave

ggsave is not really able to save images >50x50 inch, at least there are
reports of crashes and the tests fail sometimes. Reduce the limit to 25,
test only 26 in the "big size" test and hope for the best...
TST: Update baseline image for density test

The old image had some wrong dimensions (not the current ggplot default) and
so the test were failing.
TST: Update baseline images for matplotlib theme

Although the tests were not failing, the old baseline images had a different
and wrong background.
TST: Update baseline images for geom_text

The old baseline image had serif text, but that is not the default (sans serif).
TST: Add missing baseline images

These images were missing due to a failing testcase which prevented
the images to be generated and added.
TST: Visual test script: mark non existing current images as failure

The old code would assume that no produced images (due to a exception
in the code would be a "passed" test. Now, a non-produced images marks
the test as not passed in the html output of the visual test.
TST: Split test_basic in smaller tests