cleanup: Change Salty to Brackish

In the code, the difference between SALTYWATER and SALTWATER is hard
to see. More importantly, in the UI - Brackish is the word for water
that has more salt that freshwater but less salt that seawater. The
docs already use the word to clarify what is meant.

core/qthelper.h

@@ -11,7 +11,7 @@ struct picture;
 // 1) Types
 
 enum inertgas {N2, HE};
-enum watertypes {FRESHWATER, SALTYWATER, EN13319WATER, SALTWATER, DC_WATERTYPE};
+enum watertypes {FRESHWATER, BRACKISHWATER, EN13319WATER, SALTWATER, DC_WATERTYPE};
 
 // 2) Functions visible only to C++ parts