New preferences

With the new options from the new libcangjie, we need a new design for the preferences.

I'm not super happy with this one, but I can't think of anything better. I've been trying various iterations of it with some users, and this seems to be the « least worse » so far.

Hopefully we can make it better in the future. :-)

This pull request also ports our settings management to GSettings, instead of the (soon-to-be) deprecated IBusConfig.