Fix icon issue under non KDE environment

Non-KDE desktop environment like DDE and OS like Windows might not have the correct icon to display. This patch fixes this issue.

Before the fix (screenshot with the KDE official release downloaded from https://cdn.kde.org/ci-builds/office/, Currently marknote-master-731-windows-cl-msvc2022-x86_64.7z:

After the fix (screenshot under Windows, build with MSYS2's UCRT toolchain):

See also:

• frameworks/kiconthemes#3
• https://planet.kde.org/christoph-cullmann-2024-05-11-kde-applications-icons/

Similar MR:

• multimedia/elisa!632 (merged)

cc @aacid