Split Text: Rewrite with Support for manual kerning and SVG2 flowed text

Originially I just wanted to copy https://github.com/acardona/scripts/tree/master/python/inkscape into the module. But the code was really ugly, didn't work with transforms and nested tspans and had problems with SVG2 flowed text. This MR fixes that.

Note that this merge request uses a px-based document on purpose since the coordinate conversion functions are broken at the moment (#367 (closed)). I have a fix for that in the works, and with that fix, the new code of this extension works fine for mm-based documents as well.

• Closes #142 (closed)
• Closes #257 (closed)
• Fixes https://bugs.launchpad.net/inkscape/+bug/960013
• Fixes https://bugs.launchpad.net/inkscape/+bug/1576761
• Improves https://bugs.launchpad.net/inkscape/+bug/1585897 (but a true fix would require an entire text rendering submodule of inkex)