FloatImage: use any kwargs as CSS

[Conengmo]

Fixes #1667.

We introduced a regression in #1570 by always setting width to 100 %. At first I wanted to fix it by making the argument optional. But then, after also checking #1001 again, I opted to just open up the class to any CSS properties. With users being able to apply any CSS, we shouldn't hear about this again.

Note that this does mean an API change compared to version 0.13.0, since width will now require a unit like '%' or 'px' instead of just an integer. But I think that's a good thing. I didn't want to mess with the bottom and left parameters though, because I assume those have been around for much longer.