textLength and lengthAdjust not working on TSPAN elements
#### Summary:
SVG 1.1 defines the attributes _textLength_ and _lengthAdjust_ for TSPAN elements. However, Inkscape does not render these correctly. It does, however, seem able to use those attributes on TEXT elements.
#### Steps to reproduce:
1. create TEXT element in Inkscape, comprising of two lines (two TSPAN elements)
1. in the XML editor, click the second tspan element
1. add the following attributes:
- textLength: 500
- lengthAdjust: spacingAndGlyphs
1. OK changes
1. Save file and open in Chrome (second TSPAN displays wider)
#### What happened?
Second TSPAN element was not affected by the _lengthAdjust_ and _textLength_ attributes in Inkscape's view, but is affected by the attributes in other software (such as Chrome).

#### What should have happened?
The second TSPAN element should have been greatly wider than the first - as rendered in Google Chrome and indicated by the SVG 1.1 specification for TSPAN elements.

#### Version Info:
- Inkscape Version: 1.0 (4035a4fb49, 2020-05-01)
- Operating System: Microsoft Windows 10 Pro
- Operating System version: 10.0.16299 Build 16299
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