Rename stretch mode to scaling mode internally

It's been long overdue that this variable be named properly. 2.2 added
integer scaling mode (thanks Ethan), 2.3 renamed it to scaling mode. Now
2.4 will properly call it what it is so people won't be confused by it.

The ScreenSettings struct member is renamed from stretch to scalingMode
along with the Screen class member being renamed, as well as the
toggleStretchMode function being renamed to toggleScalingMode as well.
Unfortunately, due to compatibility, we can't change the <stretch> XML
tag.

desktop_version/src/ScreenSettings.h

@@ -9,7 +9,7 @@ struct ScreenSettings
     int windowHeight;
     bool fullscreen;
     bool useVsync;
-    int stretch;
+    int scalingMode;
     bool linearFilter;
     bool badSignal;
 };