Clipboard request causes popups in Inkscape

Migrated from: https://bugs.launchpad.net/inkscape/+bug/418242

Steps to reproduce:

• Install one of the applications that abuse the clipboard request signal (okular, xfce4-clipman-plugin, etc)
• open Inkscape
• create a shape
• copy onto the clipboard

What happened?

Inkscape pops up windows because it's trying to convert the svg to many other formats such as postscript, pdf, and many others.

What should have happened?

It shouldn't pop up windows for clipboard requests. There should be a silent flag which allows the extension to use default or last used values and any output should not be shown by Inkscape itself (only when called from the clipboard code).

Inkscape Version and Operating System:

• Inkscape Version: 0.48, 0.92, 1.0 alpha
• Operating System: Ubuntu 12.04, Ubuntu 18.04, Gentoo, etc
• Operating System version: Linux