Fixes#1120
When the parent of a view is C_WORKSPACE and the movement direction
is either MOVE_PREV or MOVE_NEXT, the code would attempt to move the
views to the next output, but swayc_adjacent_output can't accept
non-directional movement commands and causes undefined behaviour and
a segfault.
If the code is simply skipped, we end up in an infinite loop.
Instead, we can allow containers whose parent is a C_WORKSPACE take the
path that handles MOVE_PREV and MOVE_NEXT, which behaves as you would
expect.
I'm not certain that this fix is entirely correct as the desired behaviour
of move_container is not very well defined, but it seems to work.
924ed6464c