Bug 10601: Add a counter on batch items modifications

Bug 10601: Add a counter on batch items modifications

After executing a batch items modifications, a dialog is displayed above
the result table. It contains the number of items (and total fields)
which has been modified.

Note that items that are selected for modification but which do not
end up actually be changed are not reported in the final counts.

This patch adds two methiods to C4::BackgroundJob, ->set() and ->get(),
that allow background jobs to pass arbitrary data back to the client.

Test plan:
1/ Go to tools/batchMod.pl
2/ Enter a barcodes list
3/ Check/uncheck items and fill some values to apply
4/ Save
5/ The table summary will be displayed with a dialog box on top:
 XX item(s) modified (with YY fields modified)
Check that XX and YY correspond with what you expected.

Signed-off-by: Liz Rea <liz@catalyst.net.nz>
batch modification still seems to work correctly, with the helpful addition of the counter. Thanks!

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Galen Charlton <gmc@esilibrary.com>