Feature request: Assign icon to extension and display in menu

Migrated from: https://bugs.launchpad.net/inkscape/+bug/168605

Steps to reproduce:

Try to create a custom extension and assign it an icon.

What happened?

inx files currently don't offer that option.

What should have happened?

Authors of extensions should be able to define an icon.

Migration info

I think that this is a good idea. One of the problems comes with implementing it. Currently all the icons in Inkscape come from one SVG file. This helps with performance and makes the rendering consistent with people creating the icons in Inkscape. If we were to do this, we'd probably want the icons to be independent files, which makes the only reasonable approach to use RSVG... which has gotten better, so it's probably worth looking into again.

Since icons have been split into separate file, might be worth looking at this again?