Centralize error/warning/info icons

There are multiple copies of these pngs in different plugins.
Let's have one version in core, also as @2x variant.

Change-Id: Iedff1a6190a72c1947dd202ae1ee46f59f9fb13c
Reviewed-by: Kai Koehne <kai.koehne@digia.com>
Reviewed-by: Alessandro Portale <alessandro.portale@digia.com>