text-input: Add on_screen_input_provided hint

The new hint is meant to indicate that the text input already provides an on-screen means to enter data, and that using the system provided input method may not be needed.

It should be used when the client presents the user with a custom on-screen input method, like an on-screen keyboard, or perhaps a dropdown list.

The new hint is meant to address the issue when the system input method is an on-screen keyboard. Without the hint, the input method would not know that it's not needed, unless the client refrained from using the input method protocol at all.

With the hint, the input method can still be enabled, while not displaying a second on-screen keyboard. This allows for the system input method to still provide accessibility services, as well as text completion or prediction.

Based on discussion in https://gitlab.gnome.org/GNOME/gtk/merge_requests/978

Signed-off-by: Dorota Czaplejewicz <dorota.czaplejewicz@puri.sm>
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