Alignment change is only applied to a tspan instead of the text element when text is selected
#### Steps to reproduce:
<!-- Describe what you did (step-by-step) so we can reproduce: -->
- open Inkscape
- create a flowed text
- type two lines
- select all and align it at the center (text-alignment)
- save the file and view it in a browser
#### What happened?
The text is displaced.
The red dot indicates the center of the text and is at the coordinate of the first `tspan`: `x=35.9`.
| example file | Inkscape | Firefox | Chrome |
| ------ | ------ | ------ | ------ |
|  | |  |  |
#### What should have happened?
The text should not be displaced.
Eliminating the `text-anchor=middle` attribute of the tspans creates a file which is rendered identically in Inkscape and browsers:
| example file | Inkscape | Firefox | Chrome |
| ------ | ------ | ------ | ------ |
| |  |  |  |
Not sure if the Inkscape or the browsers violate the standard. Pinging @Tavmjong who probably knows more about that. In any case, this looks like an easy fix to make the rendering more consistent.
PS: The example https://developer.mozilla.org/en-US/docs/Web/SVG/Attribute/text-anchor is rendered correctly in Inkscape.
#### Inkscape Version and Operating System:
* Inkscape Version: 1.1-dev (1bfe461, 2020-02-06)
* Operating System: Windows 10
* Operating System version: 10.0.18362
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