Mark selection in text editor. Scroll error into view. Fixed settings scrollbar. Undo & redo everywhere. Text editor config

• Added user configurable text editor tab size. #41
• Added user configurable text editor font size. #42
• Added undo and redo from the Graph pane throgh Ctrl-Z & Ctrl-Y. #36
• Added undo and redo from buttons in the app bar. #37
• Fixed scrollbar on setting dialog. #45
• Added scrolling of text editor error indication into view through a button. #46
• Added automatic scrolling of text editor error indication into view. #46
• Added highlighting of nodes and edges in the text editor when selected in the graph. #35