Some filter settings are not correctly initialized (reset) when creating a new filter primitive

Migrated from: https://bugs.launchpad.net/inkscape/+bug/194408

Steps to reproduce:

• open Inkscape
• create a rectangle
• open Filter editor
• Add Turbulence filter primitive. Note that the base frequency (default) is 1, but the filter editor shows a base frequency of 0.
• Change base frequency to e.g. 0.2
• Create a new object
• Create a new filter for this object
• Add Turbulence again

What happened?

The visible filter frequency is still the default value, but the filter editor displays 0.2. There is no value for turbulence set in the XML editor.

What should have happened?

The filter effects UI is correctly initialized.

Other affected fields (I think I tried them all):

• Morphology -> Radius
• Specular Lighting. This is kind of a special case, because no Light source is defined by default, but the settings for all light sources are displayed. They disappear after selecting a light source. Creating a second filter sets the Light source back to unset, but the properties of the previous light source are still visible.
  • Kernel unit length
  • Azimuth
  • Elevation
  • Specular exponent
  • Cone angle
  • and in general, all properties of the light source

Inkscape Version and Operating System:

• Inkscape Version: 1.1-dev (6b4d57f, 2020-04-01)
• Operating System: Windows 10
• Operating System version: 10.0.18362