MDL-38367 course dndupload - various tweaks to the UI

Switched to Moodle-style dialog boxes (thanks Andrew for the pointer)
Enter button now submits the form (from within the 'name' field)
Upload button is disabled when the name is empty (if the name is required for the selected handler)
Dialog now has 'What do you want to do with this text?' and 'Name' for the text
Entire course section now has a class added when you drag over it (to enable it to be styled in the future)