Port kde4_add_app_icon to ecm_add_app_icon

Summary: Follow the same pattern of similar portings.

Test Plan:
The code compiles on a GNU/Linux system. It was not tested on the
platforms where it should show visible effects (Windows and macOS).

Reviewers: Krusader, abika

Reviewed By: Krusader, abika

Subscribers: abika

Tags: Krusader

Differential Revision: https://phabricator.kde.org/D5207