[wip] targets/generic-linux: add enableKdeApplicationCache

[NoraCodes]

Description

This option allows home-manager to trigger kbuildsycoca to refresh the KDE .desktop file cache, making desktop applications appear in menus.

Checklist

• [x] Change is backwards compatible.

• [x] Code formatted with ./format.

• [ ] Code tested through nix-shell --pure tests -A run.all or nix develop --ignore-environment .#all using Flakes.

• [ ] Test cases updated/added. See example.

• [x] Commit messages are formatted like

  {component}: {description}
  
  {long description}
  

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

Maintainer CC

[NoraCodes]

This is marked as [wip] because

• [ ] I have not yet figured out how to test this change
• [ ] There is a test failure locally that I don't understand fully

[stale[bot]]

Thank you for your contribution! I marked this pull request as stale due to inactivity. Please read the relevant sections below before commenting.

If you are the original author of the PR

• GitHub sometimes doesn't notify people who commented / reviewed a PR previously when you (force) push commits. If you have addressed the reviews you can officially ask for a review from those who commented to you or anyone else.
• If it is unfinished but you plan to finish it, please mark it as a draft.
• If you don't expect to work on it any time soon, please consider closing it with a short comment encouraging someone else to pick up your work.
• To get things rolling again, rebase the PR against the target branch and address valid comments.

If you are not the original author of the PR

• If you want to pick up the work on this PR, please create a new PR and indicate that it supercedes and closes this PR.