Feature/improve media progress

Resolves issues around cursor movement after inserting image.

Fix wordpress-mobile/WordPress-iOS-Editor#394
Fix wordpress-mobile/WordPress-iOS-Editor#301
Fix wordpress-mobile/WordPress-iOS-Editor#132
Also it seems the leading hiddenChar is set in one pleace and the trailing hiddenChar in another, which could make it difficult to track where it's being added.

I'd remove that from the previous code and add a final line of code along the lines of:

html = hiddenChar + html + hiddenChar;

I think it would help on readability / maintainability.