Add '?' as another way show help dialog

Gertty maps the <F1> key to show the help dialog. In gnome-terminal
<F1> maps to the terminals help, capturing the <F1> before it
gets to gertty.

This change adds an additional help key <?> which will also display
the help dialog.

Change-Id: I97396c517f21774f0f5c94645169c83b76af2157