Code Refactor; Localization; New Methods; Assets

Refactored code to modern Objective-C. Modernization changes include
instancetype, auto-synthesized properties, NS_ENUM usage, and more.
Localized DropboxBrowser strings by converting them to
NSLocalizedStrings. Made the updateContent and logoutOfDropbox methods
public. Moved file icons from folder to XCAssets Catalog.