Bug 23179: Add 'edit subfields' button to framework management

Bug 23179: Add 'edit subfields' button to framework management

To test:
1) Go to Administration > MARC bibliographic framework
2) On a line click Actions > MARC structure
3) Note the dropdowns for each tag are: Edit, Subfields, Delete
4) Apply patch
5) Reload and note dropdowns are: Edit tag, View subfields, Edit subfields, Delete
6) Confirm options do what is selected

Signed-off-by: Bin Wen <bin.wen@inlibro.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Lucas Gass <lucas@bywatersolutions.com>