Bug 17163: Making MARC subfields structure actions buttons

Bug 17163: Making MARC subfields structure actions buttons

To test:
1) Go to Admin -> MARC bibliographic framework
2) Go to Actions -> MARC structure for any framework
3) Click Subfields
4) Confirm Edit and Delete links show as buttons with appropriate icons
in one column called 'Actions'
5) Confirm buttons do not wrap on narrower browser
6) Confirm buttons behave 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>