setup: Rename the settings UI

This follows the GNOME recommendations:
https://live.gnome.org/AllanDay/IMEGuidelines

This is part of solving bug #14
setup: Take the full/half width support out of the settings UI

This will become a run-time property.

This is part of fixing bug #14.
Remove useless attributes
setup: Major refactoring

The settings UI is now layed out in a GtkGrid, the "version" option is
now in a GtkComboBox and the "include_sc" option a GtkSwitch.

This goes a long way towards fixing bug #14.
setup: Tweak the layout, based on Allan Day's mockup
setup: Use GtkButtonBox

The Gtk doc says that GtkHButtonBox is deprecated, so let's use the new
widget.
Handle half-width chars as a property

This should be the last piece needed for the settings redesign, and as
such fixes bug #14.
setup: Pretend to be a GNOME Control Center dialog

In GNOME, our preferences dialog is run through GNOME Control Center,
not directly (because we have NotShowIn=GNOME in our desktop file).

As such, it is important to pretend that we are a native dialog from
GNOME Control Center, so that the user doesn't feel like she is being
presented with various pieces: she should see a consistent whole.

Of course, this only applies when we are run by GNOME Control Center. It
does not change anything for users launching our preferences on other
desktop environments (e.g Unity or KDE) or running the command-line
executable directly.

This fixes bug #15.