cppcheck fixes to add const

src/CharacterColor.h

@@ -175,7 +175,7 @@ public:
     /**
      * Returns true if this character color entry is valid.
      */
-    bool isValid() {
+    const bool isValid() const {
         return _colorSpace != COLOR_SPACE_UNDEFINED;
     }
 

src/History.h

@@ -110,7 +110,7 @@ public:
     // is very unsafe, because those references will no longer
     // be valid if the history scroll is deleted.
     //
-    const HistoryType& getType() {
+    const HistoryType& getType() const {
         return *_historyType;
     }