Add Select2 like Search to All Selectmenu's and Select Callback Filters

[TheWitness]

Feature Request

Is your feature request related to a problem? Please describe

Put the Select2 emulation for all selects into layout.js so that it's possible for all themes. Also extend this to the drop_callback() function and consider a CSS change for the multi-select plugin.

Describe the solution you'd like

Make select menus easier to use.

Describe alternatives you've considered

Use the select2 plugin.

Additional context

It's easier to just hook jQueryUI's selectmenu widget using the widget factory.

[TheWitness]

@browniebraun, let me know if you want to do this or you can delegate to me. Your call.

[TheWitness]

@browniebraun, I was looking at the drop-callback JavaScript function, and it's a bit of a train wreck. I think we can re-engineer it using the widget factory as you have done. Any chance you can put some time into that? All the changes would be limited to the layout.js I think.