Use QString literals

This patch covers src/gui and some leftovers from previous commit.

src/gui/properties/speedplotview.cpp

@@ -94,7 +94,7 @@ namespace
         // check is there need for digits after decimal separator
         const int precision = (argValue < 10) ? friendlyUnitPrecision(unit) : 0;
         return QLocale::system().toString(argValue, 'f', precision)
-               + QString::fromUtf8(C_NON_BREAKING_SPACE)
+               + C_NON_BREAKING_SPACE
                + unitString(unit, true);
     }
 }