Fix roundtrip conversion of line height between rich text and svg

Basically, we are inventing a bicycle here. All this recursive
parsing of SVG tree is already implemented in KoSvgTextShape.
Here we just write it once again with new bugs.

Still TODO:

1. Relative line height should be calculated as an average of two line heights, but not just the latest line height.
2. The mnemonic for starting a new line should be not just dy != 0, but (dy != 0 && dx == 0)