The input method can assign a negative value to
clutter_input_method_delete_surrounding() to move the cursor to the left.
But Wayland protocol accepts positive values in delete_surrounding() and
GTK converts the values to the negative ones in
text_input_delete_surrounding_text_apply().
https://gitlab.gnome.org/GNOME/mutter/issues/539
This allows input methods to inject key events with specific keyval/keycode,
those events will be flagged with CLUTTER_EVENT_FLAG_INPUT_METHOD so they
won't be processed by the IM again.
https://gitlab.gnome.org/GNOME/gnome-shell/issues/531
ClutterInputFocus is an abstract object to be subclassed by UI actors and
the wayland interface and represents the user of an input method. It
represents the current focus of the input method, so all emitted signals
and public API hooks are expected to be called when the input method is
currently interfacing with the input focus.
ClutterInputMethod is an abstract class (to be implemented in the upper
layers) that represents the input method itself. Besides focus management
itself, all public API calls that would be called by the subclasses are
delivered through the current input focus.
