Queue error messages so that a currently shown error message is not replaced until...

Queue error messages so that a currently shown error message is not replaced until the user confirmed the reading. If there are no pending error messages and a timeout has been exceeded, the currently shown error messages automatically fades out as soon as the user triggered another action. This behavior should correspond to Ellens suggestions (I hope :-)).

svn path=/trunk/playground/utils/dolphin/; revision=628905