Adds a new FieldDescriptor struct

Use the field descriptor struct to add fields instead of the overloads
Adds a visibility function to control whether a field is visible in the inspector

EG:
ADD_FIELD("soundFile", TypeAssetLooseFilePath, Offset(mSoundFile, SoundAsset))
            .doc("Path to the sound file.")
            .elements(SFXPlayList::SFXPlaylistSettings::NUM_SLOTS)
            .onSet(&_setSoundFile);

the grammar can change to make these easier to work with, review changes carefully

Engine/source/gui/editor/guiInspector.h

@@ -90,6 +90,8 @@ public:
          return NULL;
    }
 
+   void updateVisibility();
+
    S32 getComponentGroupTargetId() { return mComponentGroupTargetId; }
    void setComponentGroupTargetId(S32 compId) { mComponentGroupTargetId = compId; }