Fix wrong context help if tool tip was not shown for focus widget

The tool tip never has focus, so it cannot become the IContext that is
checked for context help. So, integrate the help id into Utils::ToolTip
and check the tool tip first when checking for context help.
As a side effect the [F1] button and help id for the tool tip is now also
available for use outside of the text editors.

Task-number: QTCREATORBUG-5345
Change-Id: Id975703caf161d1183c247e8ad8bb693b90fd306
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@theqtcompany.com>
Reviewed-by: David Schulz <david.schulz@theqtcompany.com>