Help: Fix check if help viewer currently has focus.

Fixes a little issue that after opening context help, clicking into the
context help viewer, and pressing F1, the help viewer would show "No
documentation available" instead of opening help mode.

Change-Id: I071c92946214680e82b1f2982b62cc909a272ad5
Reviewed-by: Eike Ziller <eike.ziller@digia.com>