Fallback to the IBus.Property.new() constructor

IBus 1.5.x introduced some very nice GI overrides for its classes, which
make the API much more Python-friendly.

I had refrained from using them though, because I wanted to keep the
compatibility with Python 1.4.2 as it is still used on (at least) Ubuntu
13.04 and Debian Sid.

However, when I worked on the IBus.Property stuff, I didn't pay enough
attention and used the overridden constructor, which doesn't exist on
IBus 1.4.2.

This fixes issue #23.