Show siblings in navigation when currentFolderOnly is not set.

**Motivation**
Depending on how a website is structured the current navigation behavior is suboptimal because it leads to an empty navigation if the current container has no children. This increases the clicks and page loads when navigation through a page.
Since the default Plone navigation portlet has an option for whether siblings should be shown this option should be respected.

**Change**
When the option "Only show the contents of the current folder." in the navigation portlet configuration is not activated, show also siblings in the navigation. Activating this option results in the "old" navigation behavior.

**Updating**
This change means that existing navigation portlets might need to be reconfigured when the siblings should not be shown, since the default value of this option is to show siblings.
I did not include an upgrade step because I think when updating a website with this change the decision whether to activate or deactivate the option has to be made anyway.

**Tests Included**
I've included browser tests for the navigation before implementing this change for making sure that it does not brake existing behavior.

**Other Options**
If we do not include this change it is important that the current renderer, which is quite special and not always optimal, can be deactivated.
The problem is that the default Plone renderer output is not styled at all and should be in this case.

@ninfaj could you take a look at this change?
/cc @maethu