'C-g' should cancel long keybinding in progress (if any).

Quick fix for issue hiro-sun#30.
Known issues:
1. Generally, 'C-g' should be pressed twice. This is connected with curios
current behavior: whenever there is an active selection, pressing 'Ctrl+g'
for the first time clears up the selection, but extension does not receive
neither 'C-g', nor 'Type' command. Pressing 'Ctrl+g' for the second time
generates 'C-g' command.
2. Unlike Emacs, this extension does not preserve current selection on
pressing 'Ctrl+g'.
TODO:
Investigate this situation in more details. Is it VSCode designed behavior?

src/editor.ts

@@ -26,6 +26,12 @@ export class Editor {
         });
     }
 
+    onCancel() : void {
+        this.keybindProgressMode = KeybindProgressMode.None; // cancel long keybinding in progress (if any)
+        this.setStatusBarPermanentMessage("");
+        this.setStatusBarMessage("Quit");
+    }
+
     setStatusBarMessage(text: string): vscode.Disposable {
         return vscode.window.setStatusBarMessage(text, 1000);
     }

src/operation.ts

@@ -43,7 +43,7 @@ export class Operation {
                 this.editor.setStatusBarMessage("Undo!");
             },
             'C-g': () => {
-                this.editor.setStatusBarMessage("Quit");
+                this.editor.onCancel();
             },
             "C-x_r": () => {
                 this.editor.setRMode();