Rename "event" to "message"

After the recent refactorings, a "control event" is not necessarily an
"event" (it may be a "command"). Similarly, the unique "device event"
used to send the device clipboard content is more a "reponse" to the
request from the client than an "event".

Rename both to "message", and rename the message types to better
describe their intent.

app/src/device_msg.h

@@ -0,0 +1,32 @@
+#ifndef DEVICEMSG_H
+#define DEVICEMSG_H
+
+#include <stdbool.h>
+#include <stdint.h>
+#include <unistd.h>
+
+#define DEVICE_MSG_TEXT_MAX_LENGTH 4093
+#define DEVICE_MSG_SERIALIZED_MAX_SIZE (3 + DEVICE_MSG_TEXT_MAX_LENGTH)
+
+enum device_msg_type {
+    DEVICE_MSG_TYPE_CLIPBOARD,
+};
+
+struct device_msg {
+    enum device_msg_type type;
+    union {
+        struct {
+            char *text; // owned, to be freed by SDL_free()
+        } clipboard;
+    };
+};
+
+// return the number of bytes consumed (0 for no msg available, -1 on error)
+ssize_t
+device_msg_deserialize(const unsigned char *buf, size_t len,
+                       struct device_msg *msg);
+
+void
+device_msg_destroy(struct device_msg *msg);
+
+#endif