Multiple graphs in local storage + DOT source through URL + bug fixes

Added

• Export as URL. Generates a link to the application with the DOT source code as an URL parameter.
• Specification of the DOT source code through a URL parameter. #69
• Disabling of the undo and redo buttons when there is nothing to undo or redo.
• Allow multiple graphs to be stored in the browser's local storage. #70
  • Name and save a graph to local storage
  • Open a named graph from local storage
  • Allow sorting graphs on name, DOT source and last modification time in the open from browser dialog
  • Allow deleting graphs in the open from browser dialog
  • Show graph thumbnails and allow preview in the open from browser dialog
  • Create new empty graph
  • Rename current graph

Fixed

• Ctrl-Y and Ctrl-Z descriptions are missing in the keyboard shortcuts help dialog. #90
• If the DOT source is cleared when an error is indicated in the text editor, the old error message is still displayed. #88
• When the DOT source is cleared in the text editor, the old graph is still visible. #87
• The error button in the text editor might be covered by the highlighting of the current line. #85
• Corrected size of GitHub icon in app bar.
• When inserting a node with default shape by click in the node shape insert panel, the node gets an incorrect shape. #77
• The selection indication in the graph is cleared when a node is inserted. #78
• The graph pane is not focused after inserting a node shape from node shape insert panel. #58