Using `else` after `return` is not necessary, and can often lead to unnecessarily cluttered code. By using the `no-else-return` rule in ESLint we can avoid this pattern, see http://eslint.org/docs/rules/no-else-return.
This patch:
- Simplifies the way fields are passed to the document properties overlay
- Simplifies the way fields are filled internally in the document properties overlay
- Avoids passing a document properties reference to the secondary toolbar
This patch:
- Puts public methods at the top of the file
- Puts private methods below the public methods and marks them with an underscore
- Adds JSDoc comments to the class
- Adds setDocumentAndUrl to avoid having to handle that in `viewer.js`
