This introduces the following `for_each` functions:
* root_for_each_workspace
* root_for_each_container
* output_for_each_workspace
* output_for_each_container
* workspace_for_each_container
And introduces the following `find` functions:
* root_find_output
* root_find_workspace
* root_find_container
* output_find_workspace
* output_find_container
* workspace_find_container
* container_find_child
And removes the following functions:
* container_descendants
* container_for_each_descendant
* container_find
This change is preparing the way for demoting sway_container. Eventually
these functions will accept and return sway_outputs, sway_workspaces and
sway_containers (meaning a C_CONTAINER or C_VIEW).
This change also makes it easy to handle abnormalities like the
workspace floating list, root's scratchpad list and (once implemented)
root's saved workspaces list for when there's no connected outputs.
Ryan Dwyer
Brian Ashworth
Drew DeVault
Frantisek Fladung
Zandr Martin