CC-5450 : Refactor Media Management (Classes/DB) in Airtime

fixed column sorting problem
CC-5450 : Refactor Media Management (Classes/DB) in Airtime

working on new query for prev/current/next
CC-5450 : Refactor Media Management (Classes/DB) in Airtime

new code for getting the dashboard json.
CC-5450 : Refactor Media Management (Classes/DB) in Airtime

creating service function for column datatables settings script.
context menu on right click, dblclick event now used for adding to shows/playlist.
Merge branch '2.6.x' of https://github.com/sourcefabric/Airtime into 2.6.1