tree/container: introduce `container_is_sticky[_or_child]` functions

To query whether a container is sticky, checking `con->is_sticky` is
insufficient. `container_is_floating_or_child` must also return true;
this led to a lot of repetition.

This commit introduces `container_is_sticky[_or_child]` functions, and
switches all stickiness checks to use them. (Including ones where the
container is already known to be floating, for consistency.)

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