Fix for adding panel to panel group via plugin

When adding panels to panel group, it should also be added to the
list of panels at the class atttribute. Otherwise, it will lose the
reference to the panels when _autodiscover() is triggered.

Change-Id: Ifa530d5a39f48d9fd9cd0ea66f6c16594447058b
Closes-Bug: #1329050
Merge "Fix for adding panel to panel group via plugin"