gui/macapp: make submenu of aliases for 'webopen'

gui/macapp: make submenu of aliases for 'webopen'

this changes the 'open webroot' menu item to be a submenu listing all aliases
defined in ~/.tahoe.  Note that the dock menu does not support submenus, so it
only offers a single 'open webroot' option for the default tahoe: alias.

I had trouble with this at first and concluded that the submenus didn't work,
and made it a distinct 'WebUI' menu in it's own right.  on further inspection,
there are still problems but they seem to be something like once the dock menu
has been used, sometimes the app's main menubar menus will cease to function,
and this happens regardless of whether submenus or plain simple menus are used.
I have no idea what the peoblem is, but it's not submenu specific.