BorderContainer: style gap between panes for splitter=false

[bill]

When splitter=true

<div dojoType="dijit.layout.ContentPane" region="top" .. splitter="true">

...BorderContainer leaves a gap between the panes which can be grabbed and dragged in order to resize the pane:

For styling reasons (see #6438 and #6764), need to support (an optional) similar gap even when pane's size is not adjustable (ie, when splitter="false"). Actually, a gap should be the default, to match the padding that nonken will be adding to BorderContainer.

But of course don't display the thumb icon if pane isn't resizable.

As usual w/dijit everything needs to be controlled via CSS rules but not sure exactly what the best way to do this is... could have a .dijitNonMovableSplitter class or perhaps just leverage the padding setting from the BorderContainer's DOM node.