make i3bar use libi3’s root_atom_contents()

This removes code duplication, which will be useful for a subsequent
commit.

Furthermore, we now don’t open X11 connections unnecessarily in some
corner cases.
Remove-child callback skips output content cons

Every container 'above' (in the hierarchy) the workspace content should
not be closed when the last child was removed.

Add a check for output content containers and do not handle them. These
cons are at the root of the output container with dockarea siblings.
They may be run through this callback when an output is killed with
RandR.

Fixes an issue that caused content cons to become urgent when the output
is killed with RandR.

fixes: #1121
Merge branch 'fix-urgent-crash'
Merge branch 'master' into next