Refactored name for Fattal / Alpha & Beta + updated ranges (see #4168)

Ranges for Threshold and Amount is now -100 / +100 integer. Effective
range should be modified in ImProcFunctions::ToneMapFattal02 if ever.

rtgui/paramsedited.h

@@ -368,8 +368,8 @@ public:
 class FattalToneMappingParamsEdited {
 public:
     bool enabled;
-    bool alpha;
-    bool beta;
+    bool threshold;
+    bool amount;
 };