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.c

@@ -0,0 +1,48 @@
+#include "device_msg.h"
+
+#include <string.h>
+#include <SDL2/SDL_assert.h>
+
+#include "buffer_util.h"
+#include "log.h"
+
+ssize_t
+device_msg_deserialize(const unsigned char *buf, size_t len,
+                       struct device_msg *msg) {
+    if (len < 3) {
+        // at least type + empty string length
+        return 0; // not available
+    }
+
+    msg->type = buf[0];
+    switch (msg->type) {
+        case DEVICE_MSG_TYPE_CLIPBOARD: {
+            uint16_t clipboard_len = buffer_read16be(&buf[1]);
+            if (clipboard_len > len - 3) {
+                return 0; // not available
+            }
+            char *text = SDL_malloc(clipboard_len + 1);
+            if (!text) {
+                LOGW("Could not allocate text for clipboard");
+                return -1;
+            }
+            if (clipboard_len) {
+                memcpy(text, &buf[3], clipboard_len);
+            }
+            text[clipboard_len] = '\0';
+
+            msg->clipboard.text = text;
+            return 3 + clipboard_len;
+        }
+        default:
+            LOGW("Unknown device message type: %d", (int) msg->type);
+            return -1; // error, we cannot recover
+    }
+}
+
+void
+device_msg_destroy(struct device_msg *msg) {
+    if (msg->type == DEVICE_MSG_TYPE_CLIPBOARD) {
+        SDL_free(msg->clipboard.text);
+    }
+}