This file contains the constructors and destructors for the GUI part of your program. You might take a look here to see what the code looks like. This file contains your constructors, destructors, signal handlers, custom functions etc. Take them out to other files, it would not matter. Though add these files to the Makefile.
Signal handlers react to user's actions.
Writing a program in this manner is not very difficult. Add the corresponding class declaration to your header file. Add the handler declaration to the corresponding class. Other changes should not affect this file. Add the handler stub and fill it.