ui: added setting for text encoding to preferences (issue #500)

* also fixed offset of source locations behind multibyte characters

src/lib/settings/ApplicationSettings.h

@@ -32,6 +32,9 @@ public:
 	int getFontSize() const;
 	void setFontSize(int fontSize);
 
+	std::string getTextEncoding() const;
+	void setTextEncoding(const std::string& textEncoding);
+
 	FilePath getColorSchemePath() const;
 	void setColorSchemePath(const FilePath& colorSchemePath);