Fix logic inversion in the keyboard input toggle action.

Plus some more session/terminal property D-Bus scaffolding - I think
for 3.0 I'll move to exposing them as objects on D-Bus rather than
the flat API approach. Flat used to be nice and simple, but that was
before they grew all these properties. OTOH, while it's more work
from my end this way, the API is mostly used by shell scripters,
and a flat API is definitely easier to use from shell script than
enumerating objects.