Use QString::toLatin1() rather than ::toAscii()

The toAscii (and fromAscii) are removed from Qt5 in favor of Latin1.

src/core/Tools.cpp

@@ -141,7 +141,7 @@ QString imageReaderFilter()
             }
         }
 
-        formatsStringList.append("*." + QString::fromAscii(format).toLower());
+        formatsStringList.append("*." + QString::fromLatin1(format).toLower());
     }
 
     return formatsStringList.join(" ");