[GUI] ensure connection error toolbar button is always displayed

The connection state ERROR will immediately followed by connection state
DISCONNECTED and so this icon will almost seen for a percentage of a
second. Also replaced the images. 

We have so many (unused) images or images that were just C&P + renamed
for no reason. Should now look nicer than before.

Change-Id: I9117a167f43b0c1d62fd042b68704b46f8f6e27a
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1128
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>