Add a "separator" parameter to the drawer extension points

To add a separator before an extension point in the drawer, the UIX needs to manually call #drawerSeparator(), like the following example:

{{velocity}}{{html clean="false"}} ## we need clean="false" because we want to display the raw content of #drawerItem()
 #template('drawer_macros.vm')
 #drawerSeparator()
 #drawerItem('someURL', 'someIcon', 'some xml-escaped text', 'some HTML id')
{{/html}}{{/velocity}}

It has the drawback that if there is no item before the UIX, the displayed separator is useless.

We could have other problems like 2 uix defining manually a separator, but one of them do not display content because of an if, so at the end we have 2 separators in a row, etc...

A better solution, as implemented in XWIKI-12636, is to introduce a separator parameter in the UIX and let the skin manage the separators (displaying it or not depending on the context).

It will also be consistent with XWIKI-12636.