svg1.2: flow elements use non-existent attribute value text-align="middle"

[carlosame]

In SVG, attributes based on CSS properties are supposed to borrow their values from that property's valid values. However, the (deprecated) SVG 1.2 flow elements have a text-align attribute based on the CSS text-alignproperty, for which they use the non-CSS attribute value text-align="middle" while at the same time they do not accept the CSS-valid center.

The SVG 1.2 flow elements are no longer part of SVG and only Batik and Inkscape support them, however for consistency they should support text-align="center" in addition to text-align="middle" (preserved for Batik compatibility).