Extend the git GUI by a text editor

[eugenk]

As a follow-up on #57, add an editor for create, show, edit of files. Investigate which editor is the most suitable:

• Ace
• Codemirror
• something different

Important features:

• In-place switching of view-mode and edit-mode
• (Custom) Syntax highlighting - Maybe even this much customised that markers like (blue+bold + red waved underline) can be placed in the text
• Inline highlighting of errors/warnings given a line/column range
• Pop-up display of errors/warnings (when an inline-highlight of an error is hovered/clicked)