Media panel work

[Antranig Basman]

I've pushed to michelled's FLUID-4607 branch of infusion some work for the media panel for UIOptions. This 
takes care of all the work purely on the UIOptions side - that is, the work not specific to integrating with 
the Mammals demo. There has been:

Fully linked up media panel with UIOptions model.

Added language setting to panel.

Refactored fluid slider control so that slider portion is fully operational standing alone, taking care of 
its own ARIA settings.

Some reorganisation of options structure so that slider options are surfaced out to top level of UIOptions.

Fix to UIEnhancer (Store) so that unknown options are merged in rather than appearing as empty until reset.

Broken out some media panel-specific stying into its own CSS file preparing for one day factoring this off 
completely.

Added some new styling classes to aid consistency of layout across the other panels.


The UI for the language setting differs slightly from that in the wireframes, since I thought it would be 
more straightforward to have one default language setting for captions and transcripts, even though both are 
independently selectable in the player. We can fix up this decision later when we revisit the design.

https://github.com/michelled/infusion/tree/FLUID-4607


I'll tackle the demo integration and options broadcasting work tomorrow (Monday)

Antranig