Box.Filler components should be unfocusable by default

Box.Filler components are created by Box methods that return struts and glue. Their purpose is to control layout. To the user, they are blank space with no semantics.

Currently, Box.Filler instances are focusable by default. As a consequence, they become members of the implicitly created focus traversal policies, which means that their existence as components can be seen by users. This is an easy mistake to make. It can be avoided by making these components unfocusable by default.