some clean-up for "elementInformations" in element-file

- for some time now and for whatever reason, element-editor
  sometimes adds element-information without content –> do not
  save info-lines without any content
- sort element-information alphabetically by name in element-file
- use trimmed strings for element-information to remove leading
  and trailing whitestpace

sources/diagramcontext.h

@@ -99,7 +99,7 @@ class DiagramContext
 	
 		void add(DiagramContext other);
 		void remove(const QString &key);
-		QList<QString> keys(KeyOrder = None) const;
+		QList<QString> keys(KeyOrder = Alphabetical) const;
 		bool contains(const QString &) const;
 		const QVariant operator[](const QString &) const;
 		bool addValue(const QString &, const QVariant &, bool show = true);