QPlatformInputContextPlugin
Gtk.IMContext
zwp_input_method_context_v1
zwp_input_method_v2
xim

zwp_text_input_v3?
send backend: zwp_virtual_keyboard_v1?

## Scheme

Note: I'm not a Scheme professional, this is an experiment, feel free to
suggest improvements.

Concepts:

- Layout - a procedure that returns a UI layout


API:

popup.setLayout(layout: Layout | null)

keyboard.setLayout(layout: Layout | null)


TODO: multiple inheritance, diamond problem
(it's technically allowed but the semantics aren't very good)

MAYBE PLANNED: associated data (currently classes instances are
singletons, which really simplifies ffi)

NOT PLANNED: dynamic classes (i.e. updating classes after their
definition). You can probably hack something up if you really need it.