Automatically add enable-background="new" when new/edited filters need it.

Summary:

Seems like I can't figure out what the Blend filter does and/or it has some problems. In general, I can't figure out how to make any filter access the "background image" successfully. The manual documentation for getting "enable-background" set Update: it turns out a restart was needed after setting enable-background="new".

Steps to reproduce:

• open Inkscape with empty profile directory.
• Make a "background" by drawing a rectangle (should get filled blue by default)
• Make an "image" by drawing another rectangle inside the first rectangle
• For visibility, change color of this new rect to green and give radial gradient for fill
• Select Filters->Fill and Transparency->Blend...
• Note that Source defaults to "Background" and Mode defaults to "Multiply"
• Click Live Preview

What happened?

Filter appears to produce no changes in the display.

What should have happened?

Seems like something should have been changed by the filter. Switch between all of the available Modes one by one and none of them produce any changes in the display.

Version Info:

- Inkscape Version: 1.0.2 (e86c870, 2021-01-15)
- Operating System: Windows
- Operating System version: 10