Convert immodules to use an extension point
Add an extension point called gtk-im-module, which requires the type GtkIMContext. Simplify the loading by using GIO infrastructure. We drop the locale filtering for now, I don't think it is really necessary nowadays.
Convert most existing platform modules to by gio modules. The quartz one still needs to be done. Also still to do, perhaps: Drop the conditional build machinery for including. Either always include them, or never.