Bug 17162: Moving MARC tags structure actions into a drop down menu

Bug 17162: Moving MARC tags structure actions into a drop down menu

To test:
1) Go to Admin -> MARC bibliographic framework
2) Go to Actions -> MARC structure for any framework
3) Confirm that three links, Subfields, Edit and Delete, now show in
drop-down menu with appropriate font awesome icons
4) Confirm that Subfields takes you to the right page and Edit and
Delete work as expected

Sponsored-by: Catalyst IT
Patch behaves as expected.
Signed-off-by: Marc Véron <veron@veron.ch>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>