de::App|Refactor: More explicit "GUI enabled" argument

It is a good idea to avoid plain bools in APIs for improved
code readability.

doomsday/engine/portable/src/dd_init.cpp

@@ -133,7 +133,7 @@ int main(int argc, char** argv)
     }
 
     // Application core.
-    de::App dengApp(argc, argv, useGUI);
+    de::App dengApp(argc, argv, useGUI? de::App::GUIEnabled : de::App::GUIDisabled);
 
     // Override the system locale (affects number/time formatting).
     QLocale::setDefault(QLocale("en_US.UTF-8"));

doomsday/libdeng2/include/de/core/app.h

@@ -46,8 +46,13 @@ namespace de
         /// The object or resource that was being looked for was not found. @ingroup errors
         DENG2_ERROR(NotFoundError);
 
+        enum GUIMode {
+            GUIDisabled = 0,
+            GUIEnabled = 1
+        };
+
     public:
-        App(int& argc, char** argv, bool useGUI);
+        App(int& argc, char** argv, GUIMode guiMode);
 
         /**
          * Initializes all the application's subsystems. This includes Config and FS.

doomsday/libdeng2/src/core/app.cpp

@@ -29,8 +29,8 @@
 
 using namespace de;
 
-App::App(int& argc, char** argv, bool useGUI)
-    : QApplication(argc, argv, useGUI),
+App::App(int& argc, char** argv, GUIMode guiMode)
+    : QApplication(argc, argv, guiMode == GUIEnabled),
       _cmdLine(argc, argv),
       _config(0)
 {