Add forward class declarations to header files

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.

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.

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 un-check this option under Preferences (⌘+,) => Converter and toggle the corresponding checkbox: