kdeconnect: make package configurable

[Briaoeuidhtns]

Description

kde 6 is available in unstable and there didn't seem to be a way to change to the kde6 version of kdeconnect. Not sure if just exposing package is the best way to do this or if there's something more preferred

Checklist

• Change is backwards compatible.

• Code formatted with ./format.

• Code tested through nix-shell --pure tests -A run.all or nix develop --ignore-environment .#all using Flakes.
  firefox-profile-settings fails before and after my change, seems to be fixed by firefox: update expected container settings #5147

• Test cases updated/added. See example.

• Commit messages are formatted like

  {component}: {description}
  
  {long description}
  

  See CONTRIBUTING for more information and recent commit messages for examples.

• If this PR adds a new module

  • Added myself as module maintainer. See example.

Maintainer CC

@adisbladis