Add a tooltip to inform the users that the show/hide buttons also reset the panels width

Since clicking the show/hide panel button implemented through XWIKI-12824 it also resets the panel width, we should inform the users (e.g. through the button tooltip) of this aspect.

This may be an usability issue where an user defines a custom panel width, clicks the hide button, then expects the panel to have the same custom width after it shows back.

It may be implemented like:

Toggle the right panel column. This also resets the panel's width.