Add tooltip to Object Properties clarifying that the Label field is lost during the plain svg export

Migrated from: https://bugs.launchpad.net/inkscape/+bug/1811608

Steps to reproduce:

• open Inkscape
• open any inkscape svg file with objects
• add labels to objects using object properties
• save as plain svg
• open the svg back up

What happened?

• object dialog and object lists have lost their labels, replaced with instead a random id label.

What should have happened?

• It should have saved a custom label into the id field to preserve some of the content

Inkscape Version and Operating System:

• Inkscape Version: 1.0 Alpha
• Operating System: Ubuntu
• Operating System version: 18.04