public (konstantin.scheglov@gmail.com):
This patch contains:
1. tweak for LengthAttributeParser to support +/- before value;
2. check for negative width/height in LayoutPanelParser;
3. tests for negative values in LayoutPanel.

Review by rjrjr@google.com
http://gwt-code-reviews.appspot.com/1094801

Review by: sbrubaker@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@9209 8db76d5a-ed1c-0410-87a9-c151d255dfc7