fix(mac): show local help in app

Fixes #3979.

Instead of loading help.keyman.com in the support frame, we now load a
local copy of the help. External links within the help will open in the
user's browser. This avoids issues of navigation to keyboard download
and other sites which won't work well.

Also, adds Back, Forward and Home buttons to the window so the user can
navigate more easily with mouse.

Finally, this means that the local help will always be synchronised with
the app version and brings the macOS help in line with the other
platforms.