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/mouse/juce_MouseCursor.cpp

@@ -75,7 +75,7 @@ public:
     MouseCursor::StandardCursorType getType() const noexcept    { return standardType; }
 
 private:
-    CustomMouseCursorInfo info;
+    detail::CustomMouseCursorInfo info;
     PlatformSpecificHandle handle;
     const MouseCursor::StandardCursorType standardType;
     const bool standard;