keep the hterm iframes in their own container

We can't put the hterm iframes in the actual view-manager hierarchy
because they won't servive being reparented.  Instead, they're kept
in a separate place, and moved over the correct position/size as
necessary.
initial add of hterm executable

Switch the launch-app command to run axiom_shell/exe/hterm.  If you
invoke it again, you get another terminal window.