make Paragraph use CairoText

[behackl]

Motivation

In some instances, Pango and Text works in a weird way due to the fact that it does some things in a rather different way when compared to Cairo and CairoText. The class Code is one of those that relied on certain behavior of CairoText that changed when switching to Pango -- and has not yet been fixed. In order to have a working version of Code again, this PR makes Paragraph use CairoText instead of Text and thus restores the old behavior.

Explanation for Changes

One thing that was changed in particular was how whitespace is counted: Text currently does not add invisible mobjects for spaces, which is why the coloring that Code applies to a given code snippet is off.

Testing Status

With this PR, Code("snippet.py", background="window", font="Roboto Mono") renders as this:

Acknowledgement

• I have read the Contributing Guidelines

[behackl]

I could also add the changes from #476 and some additional changes to make sure the canvas size for CairoText is set dynamically based on the width and height of the passed string. (This caused problems for Pango, unfortunately, otherwise we could have merged #476...)