Allow to define the default behavior of a panel: collapsed or expanded

self explanatory (I think )

Actually it's not for me There are various ways to define this:

1. Add the ability to collapse/expand panels
2. Sub-option: make the state permanent (using a cookie for example)
3. It could also mean to have an Admin UI option to decide how each panel should be presented initially: either expanded or collapsed.

I'll assume that the need was 1) and not 3), with 2) being an option.

This issue was a duplicate of XWIKI-10498 for which it was decided to close it as "won't fix". Instead, we decided to go with XWIKI-12824. Thus, closing this issue as "Won't fix" too.