Sortable Multichoice field

[kunalgrover05]

I have modified the default form widget to enable sorting of data which is added into our DB. This PR makes this behavior default since I am not sure if there is any disadvantage of doing that.

This adds an extra dependency of JQuery UI library: https://jqueryui.com/
Possibly, any other simple JS library can replace the JQuery UI module.

Behavior

• All list of items are sortable. Since there is no change in how the DB stuff is handled, only sorting of the selected items will be saved.
• When the form is reloaded, all selected items show on top in sorted order, and unselected items go at bottom(in the same order they are defined).
• When sorting is changed, the form is marked as dirty and saved again. This is a change from the current behavior.

PS: I am unsure of my sorting technique by overriding optgroups function. Any advise is welcome.

[tomasgarzon]

Hi, could you update from master?