Make additional segment editor effects more discoverable

[lassoan]

Is your feature request related to a problem? Please describe.

There are several extensions that provide additional effects for Segment Editor. They are all listed in the Extensions Manager in Segmentation category and have the Segment Editor icon in their logo. Still this does not make them discoverable for new users.

Describe the solution you'd like

We could add an effect with a "+" icon that would show a button that would open up the extension manager that would list all the extensions that contain segment editor effects.

Describe alternatives you've considered

Moving all effects into the Slicer core would work for some effects, but not all.

Additional context

Discussed in https://discourse.slicer.org/t/move-effects-from-segmenteditorextraeffects-into-slicer-core/19768