Make 'focus' disable blocking fullscreen windows

Make 'focus' disable blocking fullscreen windows

The problem here is that con_fullscreen_permits_focusing() does not
check if there is a blocking fullscreen container in the workspace that
the container to be focused belongs. This makes it possible to focus a
container behind a fullscreen window if it's in an unfocused workspace.

This commit introduces a change in the 'focus' command behaviour. When
focusing a container blocked by a fullscreen container, either CF_OUTPUT
or CF_GLOBAL, the blocking container loses its fullscreen mode and the
target container is focused like normal.

This should not affect directional focus commands: left, right, up,
down, parent, child.

Fixes issue #1819.