This is similar to #3820 but does not use qsort but an insertion sort in
con_attach.
Since each bar block automatically gets its own incremental bar id,
bards end up being sorted according to their definition order in the
config file.
For i3bar, the WM_CLASS is modified to include an instance name which
depends on the bar_id. This could be useful for other reason, e.g. users
targeting a specific bar instance.
Fixes#3491
7df88f18eb