Extract keyboard processor trait

This will allow to provide alternative key processors.

app/src/trait/key_processor.h

@@ -0,0 +1,29 @@
+#ifndef SC_KEY_PROCESSOR_H
+#define SC_KEY_PROCESSOR_H
+
+#include "common.h"
+
+#include <assert.h>
+#include <stdbool.h>
+
+#include <SDL2/SDL_events.h>
+
+/**
+ * Key processor trait.
+ *
+ * Component able to process and inject keys should implement this trait.
+ */
+struct sc_key_processor {
+    const struct sc_key_processor_ops *ops;
+};
+
+struct sc_key_processor_ops {
+    void
+    (*process_key)(struct sc_key_processor *kp, const SDL_KeyboardEvent *event);
+
+    void
+    (*process_text)(struct sc_key_processor *kp,
+                    const SDL_TextInputEvent *event);
+};
+
+#endif