Remove build warnings for preferences.xib

Disable "Auto Recalculates Cell Size" option, which is not supported
before OS X 10.8.
Override keyboard layout correctly. Fixes #86.

Using numerous NSLog's led to the discovery that when McBopofomo lost
function (as described in #86), -setValue:forTag:client: was often called
not just on the context of the foreground app, but also on the contexts
of the background apps. This led to the theory that calling keyboard
layout override in that method (not a documented way of doing things
anyways) might corrupt the input method context. That we swapped out
language model and the builder when the method got called didn't help.

In this commit, we put back the keyboard layout override code to where
it belongs -- in -activateServer: -- and we now only swap the language
model and re-create the builder if the input method really changes (e.g.
from Bopomofo to Plain Bopomofo, or vice versa).

Similar defensive coding is also used in the function key handler in the
-handleEvent:client: method.