Address some PR feedback.

- Better variable names
- Close a parenthesis in a comment
- Check equality against nullptr instead of 0
- Add comment to document method of clearing a QLayout
- Format some things
- Const correctness
- Remove a TODO in a comment
- Attempt at removing concatenation of string literal AZ_FUNCTION_SIGNATURE, which might not be a literal on every compiler/platform

Code/Sandbox/Editor/EditorViewportWidget.h

@@ -385,7 +385,7 @@ protected:
     };
     void ResetToViewSourceType(const ViewSourceType& viewSourType);
 
-    bool ShouldPreviewFullscreen();
+    bool ShouldPreviewFullscreen() const;
     void StartFullscreenPreview();
     void StopFullscreenPreview();