Convert immodules to use an extension point

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. Drop the locale filtering for now, I don't
think it is really necessary nowadays.

Convert existing platform modules to gio modules.
Sill to do: Drop the conditional build machinery.
Either always include them, or never.