Improved QAPPLICATION_CLASS macro

Now it can be defined in the cpp file that include it (usually
main.cpp).

README.md

@@ -42,7 +42,7 @@ Using ```QApplication::instance()``` is a neat way to get the ```SingleApplicati
 Extending from other application classes
 -----------------------
 
-```SingleApplication``` extends from the ```QApplication``` class by default , but you can easily change that to ```QGuiApplication``` or ```QCoreApplication``` from the ```singleapplication.h``` file, using the ```QAPPLICATION_CLASS``` macro.  
+`SingleApplication` extends from the `QApplication` class by default , but you can easily change that to `QGuiApplication` or `QCoreApplication` with the `QAPPLICATION_CLASS` macro. Define it before the include statement of `singleapplication.h` or change the default value in the file.
 
 __Example:__
 

singleapplication.h

@@ -2,7 +2,9 @@
 #define SINGLE_APPLICATION_H
 
 // Change this to inherit from QGuiApplication or QCoreApplication
-#define QAPPLICATION_CLASS QApplication
+#ifndef QAPPLICATION_CLASS
+  #define QAPPLICATION_CLASS QCoreApplication
+#endif
 
 #define  QUOTE(C) #C
 #define  INCLUDE_FILE(C) QUOTE(C)