Introduce get_assigned_output

Introduce get_assigned_output

This also replaces code in create_workspace_on_output() that is
theoretically more efficient but:
1. It isn't a huge difference since it depends on the number of outputs,
that shouldn't be high.
2. get_assigned_output will be modified and used for #555, then its
logic should be followed in create_workspace_on_output() too.

Another note for create_workspace_on_output: if assigned is not NULL the
condition (assigned != output->con) should never be false, ie if there
is an assigned output to this name, it isn't the current one. This
happens because the current callers check for assignments before calling
create_workspace_on_output().