setup: Redesign the preferences dialog
data: Add our default GSettings schemas
Port the engine configuration to GSettings

IBus upstream will deprecate IBusConfig, and recommends engines to
migrate to GSettings instead.

Given that we are breaking the configuration options laready between 1.0
and 2.0, it seems like the good moment to make that migration, so we
don't break user configurations again in the future.

It does require quite a lot of change, but overall it seems like a net
win, as it allows us to remove quite a bit of code.

Fixes #16
tests: Port the unit tests to GSettings
setup: Rename the source file

There is no reason to have a setup module with only a single __init__.py
file, it just makes the code look more complex than it really is.