added "Auto White Balance temperature bias" feature

This new slider in the White Balance tools allows to alter the computation of
the "auto white balance" by "biasing" it towards warmer or cooler
temperatures. The bias is expressed as a percentage of the computed
temperature, so that the resuling temperature is given by
"computedTemp + computedTemp * bias".

rtgui/wbprovider.h

@@ -25,7 +25,7 @@ class WBProvider
 
 public:
     virtual ~WBProvider() {}
-    virtual void getAutoWB (double& temp, double& green, double equal) {}
+    virtual void getAutoWB (double& temp, double& green, double equal, double tempBias) {}
     virtual void getCamWB (double& temp, double& green) {}
     virtual void spotWBRequested (int size) {}
 };