Bugfix: Insert container at the correct position on workspace-level when workspace_la...

Bugfix: Insert container at the correct position on workspace-level when workspace_layout == default

Fixes #533

The problem was that the code was always executed. While it *attaches* the new
container to the workspace container, it also sets current = NULL and thus
always appends the container instead of inserting it after the currently
focused child. So now, we just don’t execute that code at all for
workspace_layout == default.