Removes unused header file
Creates icon_service

Currently icon_service registers an icon extension.
It appends the extension to all files passes to it.
In future it would accept paths to search for icons.
Then icon_service.resolved_path() would give back full path to the icon.

This prepares for searching for correct icon location and icon extension
instead of assuming a constant png icon extension.

This commit doesn't change existing way icon extensions are managed.