Show details about uncaught NSExceptions.

Previously the code only handled C++ exceptions and reported only a
generic error for uncaught NSExceptions.

Take advantage of 64bit Obj-C runtime's interoperability of Obj-C and
C++ exceptions and handle NSException too.

See #90.
Don't use REKit on OS X 10.7.

LanguageCtrl's case-insensitive auto-competition requires REKit code,
but that throws an exception on 10.7.  Rather than trying to fix the
rather low-level REKit code to work on 10.7, give up on this small
enhancement - it's not worth the effort when the next version of Poedit
will almost certainly require 10.8+ anyway.

Fixes #90.
OS X: Fix undo-related exceptions.

Reset the undo manager's state when going to another translation.
Without this, it was possible to apply undo operations after the
underlying text changed.

"Proper" solution would be to have global undo, not just local to the
current text, but that's for a future version.