Bug 20343: Show number of checkouts by itemtype in circulation.pl

Bug 20343: Show number of checkouts by itemtype in circulation.pl

It is sometimes useful to have the number of checkouts grouped by
itemtype (to see if we're approaching the maximum number of checkouts
allowed for instance).
This patch adds this information above the checkouts table, in
circ/circulation.pl

Test plan:
1. Check out some items with various item types to a patron
2. In circ/circulation.pl, click on 'Show checkouts' button to display
   the table
3. Notice the line "Number of checkouts by item type" above the table,
   click on it
4. See that the list of item types is displayed with a count in front of
   each one
5. Compare the numbers to the checkouts in the table and verify that the
   numbers are correct

Signed-off-by: claude <claude.brayer@cea.fr>
Signed-off-by: Jesse Maseto <jesse@bywatersolutions.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>