Added selection of all nodes and edges in the graph

Resolves #13

Author: magjac

src/Graph.js

@@ -281,6 +281,10 @@ class Graph extends React.Component {
       }
       this.deleteSelectedComponents.call(this);
     }
+    else if (event.key === 'a') {
+      let components = this.graph0.selectAll('.node,.edge');
+      this.selectComponents(components);
+    }
     else if (event.key === '?') {
       this.props.onHelp();
     }

src/KeyboardShortcutsDialog.js

@@ -14,6 +14,7 @@ import TableRow from '@material-ui/core/TableRow';
 import CloseIcon from '@material-ui/icons/Close';
 
 const keyboardShortcuts = [
+  {key: 'Ctrl-A', description: 'Select all nodes and edges.'},
   {key: 'Ctrl-C', description: 'Copy the selected node.'},
   {key: 'Ctrl-V', description: 'Paste the cut/copied node.'},
   {key: 'Ctrl-X', description: 'Cut the selected node.'},