This introduces a new API on `ESCInputsManager` that makes it
possible to inject custom logic for processing input events.
An example of how to use this API will be introduced in a
separate commit as part of
https://github.com/godot-escoria/escoria-demo-game/pull/503
via a new plugin, `addons/escoria-ui-keyboard-9verbs/`,
which shows how this can be used to add support for keyboard shortcuts
to select a specific verb.
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