request: convert flowtext to svg 1.1 fallback on save as plain svg

Migrated from: https://bugs.launchpad.net/inkscape/+bug/167335

Steps to reproduce:

• open Inkscape
• load sample.svg
• save as plain-svg
• validate result using https://validator.w3.org/check

What happened?

There are multiple validation errors due to the use of flowRoot

SVG element flowRoot not allowed as child of SVG element g in this context. (Suppressing further errors from this subtree.)

In some renderers this causes rendering issues.

What should have happened?

There should be no validation errors in plain-svg files.

Inkscape Version and Operating System:

• Inkscape Version: Inkscape 1.0alpha (331007d, 2019-04-11)
• Operating System: Windows 10 x64
• Operating System version: build 17134.648

sample.svg