Make title in window/progress optional (#22)

Proposing this as of microsoft#245 (comment).
Existing discussion regarding the title: #21 (comment).

Author: Xanewok

protocol.md

@@ -1340,10 +1340,10 @@ interface ProgressParams {
   id: string;
 
   /**
-   * The title of the progress.
-   * This should be the same for all ProgressParams with the same id.
+   * Optional title of the progress.
+   * If unset, the previous title (if any) is still valid.
    */
-  title: string;
+  title?: string;
 
   /**
    * Optional progress message to display.