Expanded state is not preserved if panels are collapsed by default

XWiki.togglePanelVisibility (xwiki.js) assumes expanded is the default state and doesn't store a cookie when the panel is expanded (actually, deletes the cookie when the panel is expanded).

This causes Panels don't "remember" their state if the initial state is set to "collapsed".

(Jira Issue created after a message from Marius Dumitru; see http://markmail.org/thread/uoxdysvxntezxm6e)