Class Backend

Sets up the navigators like so:

+----------+
| Screen n <-------+
+----------+       |
                   |
+----------+       |
| Screen 2 <-------+
+----------+       |                   +---------+
                   |             +-----> Modal n |
+----------+       |             |     +---------+
| Screen 1 <-------+             |
+----------+       |             |     +---------+
                   |             +-----> Modal 2 |
            +------+------+      |     +---------+
            | __Screens__ |      |
            +------+------+      |     +---------+
                   ^             +-----> Modal 1 |
                   |             |     +---------+
                   |             |
                   |        +----+-----+
                   +--------+ __Main__ |
                            +----------+

This is required as the modal style can only be set per-navigator not per-screen.