Let RFB object make sure canvas can have focus

Let's set up things properly ourselves instead of forcing everyone
to create their canvas element just like we want it.

core/rfb.js

@@ -147,6 +147,11 @@ export default function RFB(defaults) {
     // main setup
     Log.Debug(">> RFB.constructor");
 
+    // Target canvas must be able to have focus
+    if (!this._target.hasAttribute('tabindex')) {
+        this._target.tabIndex = -1;
+    }
+
     // populate encHandlers with bound versions
     this._encHandlers[encodings.encodingRaw] = RFB.encodingHandlers.RAW.bind(this);
     this._encHandlers[encodings.encodingCopyRect] = RFB.encodingHandlers.COPYRECT.bind(this);

vnc.html

@@ -331,7 +331,7 @@
                 autocorrect="off" autocomplete="off" spellcheck="false"
                 mozactionhint="Enter" tabindex="-1"></textarea>
 
-            <canvas id="noVNC_canvas" width="0" height="0" tabindex="-1">
+            <canvas id="noVNC_canvas" width="0" height="0">
                         Canvas not supported.
             </canvas>
         </div>