Bug 17355: Add the ability to remove authorised value categories

Bug 17355: Add the ability to remove authorised value categories

This new patch set is adding a new DB field is_system to distinguish
internal categories.

The list of internal categories is in the patch "DB changes - set
is_system for categories" and can be discussed/extended.

Test plan:
0.
- Apply patch
- updatedabase
1.
- Create a new AV category
- Create an authorised value
- List the AVs for this category
- Remove all the AVs for this category
=> You see 2 buttons "Add a new authorised value" and "Delete category
'XXX'"
- Click the delete button
=> You get a popup
- Accept
=> The category is deleted

2.
- List the AV for Asort1 (internal category)
/cgi-bin/koha/admin/authorised_values.pl?searchfield=Asort1
- Delete the AVs for this category (if there is any)
=> You do not see the "Delete category" button

3.
- Create a new AV category 'foo'
- Create an authorised value
- Hit /admin/authorised_values.pl?op=delete_category&categor_name=foo
=> You get an error message (expected because the category cannot be
deleted if it has AV linked)
=> message text:
=> An error occurred when deleting this authorized value category. Check the logs.

4.
- Hit /admin/authorised_values.pl?op=delete_category&categor_name=Asort1
=> You get an error message (expected because the category Asort1 cannot
be deleted, it's an internal category.
=> message text:
=> An error occurred when deleting this authorized value category. Check the logs.

Signed-off-by: Victor Grousset/tuxayo <victor@tuxayo.net>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>