Also update TP counts on unit deletion
Save the counts after unit deletion
Also update TP counts

The calls for saving the Store and the TP have been moved to the bottom
of the function.
Add fields to store the number of units with failing critical checks
Rename TreeItem method

In the future we might want to expose counts for regular checks as well.
Retrieve critical counts directly from the model fields

This way we avoid expensive calculations and we align with the
word counts and suggestions counts way of retrieving the data for
the stats.

This partially undoes 8483d758 so get_checks() returns the data
for all kind of checks.
Calculate critical counts in refresh_stats
Update critical counts on unit deletion
Update critical counts on unit saving
Stats: Don't cache counts

This eases testing. Also since we seem not to be clearing the cache we
can't safely rely on the cache.
Update critical counts when muting/unmuting critical checks