Fill Layers: add cosmetic bits

While we wait for @dkazakov 's seminar on LoD, I added toggles to the generator list's visibility.

This puts Fill Layers' configuration dialog in feature parity with Filter Layers.

(Note: I had also added layer preview, but that belongs to filters themselves- not Filter Layers.)

Test Plan

Build Krita, add a Fill Layer, check that you can hide the list of generators with the toggle.

Formalities Checklist

• I confirmed this builds.
• I confirmed Krita ran and the relevant functions work.
• I tested the relevant unit tests and can confirm they are not broken. (If not possible, don't hesitate to ask for help!)
• I made sure my commits build individually and have good descriptions as per KDE guidelines.
• I made sure my code conforms to the standards set in the HACKING file.
• I can confirm the code is licensed and attributed appropriately, and that unattributed code is mine, as per KDE Licensing Policy.