Unset fill/stroke don't reset fill-opacity or stroke-opacity, forcing clones to inherit alpha

Migrated from https://bugs.launchpad.net/inkscape/+bug/1183400

Steps to reproduce:

• open Inkscape
• create a rectangle
• open the fill and stroke dialog
• change the fill-opacity of the rectangle to 50 then back to 100%
• Change the fill to Unset (The ? icon in the Fill and stroke dialog)
• Now make a clone of it (Alt+D) and move it to the side
• On the new clone set a new different fill color
• Now set the fill's alpha value to something different than 255

What happened?

• fill-opacity was not unset from the rectangle
  • thus clone inherited the

Analysis from OP, note the fill-opacity:1 in the "bad" file.

<!-- GOOD FILE XML -->
<rect id="good"
       style="fill-rule:evenodd;"
       width="100" height="100" />

<!-- BAD FILE XML -->
<rect id="bad"
       style="fill-rule:evenodd;fill-opacity:1"
       width="100" height="100" />

What should have happened?

• Unclear. Probably default behavior for the Unset option is to affect alpha too, but ultimately it isn't wrong for alpha to be kept as a separate property.
• Some GUI way to control fill-opacity when fill is unset might be required

Version info

• Inkscape 1.2-dev (742c897e91, 2021-11-07) Linux Mint 20
• First reported 0.48.4 r9939 Windows 7