Rectangle Markers can't be edited by Fill and Stroke Dialog

Split out from #3037 , reported by @nikolayvidenov #3037 (comment 359751646)

Steps to reproduce:

• open Inkscape
• Draw a path
• Give the path a marker via the Fill and Stroke Dialog
• Copy the path
• Draw a rectangle
• Paste Style
• Draw another rectangle (optional)
• Try to change or remove the marker via the Fill and Stroke Dialog

What happened?

• Nothing happens, except the marker changes in the Fill and Stroke Dialog (but no where else)

What should have happened?

• Rectangle markers should be changed.

Markers are allowed in SVG 2 (https://svgwg.org/svg2-draft/painting.html#MarkerStartProperty). We display it anyhow, so it should be changeable.

In SVG1.1 there is some ambiguity, see https://www.w3.org/TR/SVG11/painting.html#MarkerStartProperty

• Applies to doesn't mention rect (but does mention ‘path’, ‘line’, ‘polyline’ and ‘polygon’ elements)
• but the text above it mentions basic shapes, which includes rects https://www.w3.org/TR/SVG11/shapes.html

  ‘marker-start’ defines the arrowhead or polymarker that shall be drawn at the first vertex of the given ‘path’ element or basic shape

Anyhow, this bug doesn't occur for ellipses, so the spec isn't the deciding factor here.

Version Info:

• Inkscape 1.0 (1.0+r73+1) Ubuntu 20.04
• Inkscape 1.1-dev (433571b46d, 2020-10-15)