Bug 927862 - Implement AttentionWindow

this block is really hard to read. I think some early break could help a lot with reduce nesting
```
          this._openedInstances.delete(attention);
          if (this._topMostWindow != attention) {
            attention.close();
            break;
          }

          var candidate = null;
          if (this._openedInstances.size === 0) {
            candidate = AppWindowManager.getActiveApp();
            this._topMostWindow = null;
          } else {
            candidate = this._openedInstances.entries[0]; // or we need last?
            this._topMostWindow = candidate;
          }

          if (!candidate) {
            attention.close();
            break;
          }

          candidate.ready(function() {
            attention.close();
          });
          break;

```