Fix erratic behavior with single container child jumping outputs

This fixes a regression introduced in commit
4e88c10564ca5366c2578908f62ec56625a26718: when attempting to move the
single child of a container in the direction of another output, i3
would move the window to the output, despite the window not being at
the edge of its output, instead of moving it to its parent container.

The bug occurred because the check for moving containers across
outputs with non-default workspace layouts (issue #1603) did not
actually verify that the moved window lies at the edge of the
workspace, despite what its comment said.

Fixes issue #2466.