These are the same as seat_set_focus, but accept a specific type rather
than using nodes. Doing this adds more typesafety and lets us avoid
using &con->node which looks a little ugly.
This fixes a crash that pretty much nobody would ever come across. If
you have a bindsym for "focus" with no arguments and run it from an
empty workspace, sway would crash because it assumes `container` is not
NULL.
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