Also update TP counts on unit deletion
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