Add forward class declarations to header files

Applies to:

• Xcode Extension
• Finder Extension
• Advanced Project Converter
• Offline Converter

Included in plans:

• Cloud
• Offline Converter

After converting an Objective-C class (i.e.MyViewController) to Swift (while you still have Objective-C files left in your project), we need to ensure that your Objective-C classes still have access to the converted Swift class (or a protocol).

Header (`.h`) files that reference MyViewController will have a forward class declaration (i.e. @class MyViewController) automatically generated. This is required because you cannot import the Swift Bridging Header from a header (.h) file. Otherwise, this would create a circular referencing issue.

The same functionality also works for protocols: header files that reference a protocol (i.e. MyViewControllerProtocol) will automatically generate a forward protocol declaration (i.e. @protocol MyViewControllerProtocol).

Implementation (`.m`) files will be able to access the Swift version of `MyViewController` automatically due to the Swift Bridging Header (ProjectName-Swift.h) which is automatically imported from the Precompiled Header (ProjectName-Prefix.pch). 

The Swift Bridging Header file (ProjectName-Swift.h) is automatically generated by Xcode (rather than Swiftify) if the project compiles successfully and contains at least one Objective-C and one Swift file.

To disable automatically adding forward class declarations to header files, you can uncheck this option under Preferences (⌘+,) => Converter and toggle the corresponding checkbox: