Make the width of panel column flexible

Currently in XWiki the width of the panel column can be set to 3 fixed sizes : small, medium, and large, so, it is not possible to resize the width of the panel column beyond these fixed values.

Some users have raised the need to have a flexible panel column so that they can change the width of the panels themselves, for example, to better visualise the navigation panel entries when the page titles are very long.

Note that this kind of flexible panels is available on confluence or Jira for example. See screen capture below :

screen-capture.webm