Switching to % unit scales down stroke width
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#### Steps to reproduce:
<!-- Describe what you did (step-by-step) so we can reproduce: -->
- open Inkscape
- create a path
- open fill and stroke dialog
- go to stroke style tab
- switch units to %
#### What happened?
Stroke width got scaled down and unit didn't change.
#### What should have happened?
Unit should have changed and the stroke shouldn't have changed width.
#### Inkscape Version and Operating System:
Inkscape 1.1 (inkscape@c4e8f9ed74624bfe77c98b90f5d2a8ea0dbf1431, 2021-05-24)
GLib version: 2.66.0
GTK version: 3.24.20
glibmm version: 2.60.0
gtkmm version: 3.24.2
libxml2 version: 2.9.9
libxslt version: 1.1.34
Cairo version: 1.16.0
Pango version: 1.43.0
HarfBuzz version: 2.7.1
Poppler version: 20.12.1
OS version: macOS 11.4
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