GUI Basics: Refactor juce_gui_basics file structure

- Created a new detail namespace
- Moved shared module implementation details into the detail namespace
- Split dependencies so source files only rely on details in the detail namespace
- Removed all code from the juce_gui_basics.cpp file

modules/juce_gui_basics/application/juce_Application.cpp

@@ -83,16 +83,12 @@ bool JUCEApplication::perform (const InvocationInfo& info)
 }
 
 //==============================================================================
-#if JUCE_MAC
- extern void juce_initialiseMacMainMenu();
-#endif
-
 bool JUCEApplication::initialiseApp()
 {
     if (JUCEApplicationBase::initialiseApp())
     {
        #if JUCE_MAC
-        juce_initialiseMacMainMenu(); // (needs to get the app's name)
+        initialiseMacMainMenu(); // (needs to get the app's name)
        #endif
 
         return true;