web-app-utils: fix changing webapp icon

Currently preferences dialog in webapps changes icons by modifying the Icon= line in the desktop file instead of updating the saved icon.

This causes the new icon to disappear once the source image file is deleted/renamed and system is restarted.

To fix this situation, overwrite the app-icon.png in the profile directory with the selected file and don't change the icon path in the desktop file.

Fixes #810 (closed)