MainBar: Fix #594 by changing calling order and flag usage

The reason that the sampling rate button disappears after a
sampling rate was selected is that there's a flaw in the
chain of events:

on_sample_rate_changed() which calls
commit_sample_rate() which sets updating_sample_rate_ and calls
on_config_changed() which calls
update_sample_rate_selector() which...
...clears the list and exits because updating_sample_rate_ is set.