Stop bundling pycanberra

It came to my attention that pycanberra is LGPLv2 only, while IBus
Cangjie is LGPLv3+.

The mix of the two, bundled together might be problematic, so to avoid
any issue, dropping our copy of pycanberra is the easiest way out.

The code was already handling errors to play sound anyway (no sound
would be played, so let's generalize that to the case where pycanberra
is just not available.
Work around integer overflow in pygobject on 32 bits systems

This is causing a bug for us, where all input from the user is doubled
on 32 bits systems.

The workaround is not pretty, but it's needed until most distros ship a
fixed version. :(