Extract keyboard processor trait

This will allow to provide alternative key processors.

app/src/keyboard_inject.h

@@ -0,0 +1,30 @@
+#ifndef SC_KEYBOARD_INJECT_H
+#define SC_KEYBOARD_INJECT_H
+
+#include "common.h"
+
+#include <stdbool.h>
+
+#include "controller.h"
+#include "scrcpy.h"
+#include "trait/key_processor.h"
+
+struct sc_keyboard_inject {
+    struct sc_key_processor key_processor; // key processor trait
+
+    struct controller *controller;
+
+    // SDL reports repeated events as a boolean, but Android expects the actual
+    // number of repetitions. This variable keeps track of the count.
+    unsigned repeat;
+
+    bool prefer_text;
+    bool forward_key_repeat;
+};
+
+void
+sc_keyboard_inject_init(struct sc_keyboard_inject *ki,
+                        struct controller *controller,
+                        const struct scrcpy_options *options);
+
+#endif