Bug 14268: Use dropdown for variable types

Bug 14268: Use dropdown for variable types

The information needed to build the dropdown is already passed.
This patch uses it to build the variable type dropdown correctly.

TEST PLAN
---------
 1) Apply 14135
 2) In the staff client:
     Home -> Global system preferences -> Local use
 3) Click 'Edit' on any system preference.
 4) Note all the types listed.
 5) Click 'Cancel'
 6) Apply this patch
 7) Click '+New Preference'
 8) Enter data to have a decent 'Free' type preference.
 9) Click 'Save'
10) For each type noted in step 4 (do 'Free' again last)
    a) Click 'Edit' for the added system preference
       -- it should be the last saved variable type.
    b) Change the variable type to the next type.
    c) Click 'Save'
    d) Repeat steps (a) through (c) until all the types have
       been done.
    -- NOTE: Yes, there are other strange things here, but
             those are beyond the scope of this bug.
11) Run koha qa test tools

Signed-off-by: Indranil Das Gupta (L2C2 Technologies) <indradg@gmail.com>
Signed-off-by: Tadeasm <tadeas.moravec@gmail.com>
Signed-off-by: Jonathan Druart <jonathan.druart@koha-community.org>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>