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/tests/test_device_msg_deserialize.c

@@ -0,0 +1,28 @@
+#include <assert.h>
+#include <string.h>
+
+#include "device_msg.h"
+
+#include <stdio.h>
+static void test_deserialize_clipboard(void) {
+    const unsigned char input[] = {
+        DEVICE_MSG_TYPE_CLIPBOARD,
+        0x00, 0x03, // text length
+        0x41, 0x42, 0x43, // "ABC"
+    };
+
+    struct device_msg msg;
+    ssize_t r = device_msg_deserialize(input, sizeof(input), &msg);
+    assert(r == 6);
+
+    assert(msg.type == DEVICE_MSG_TYPE_CLIPBOARD);
+    assert(msg.clipboard.text);
+    assert(!strcmp("ABC", msg.clipboard.text));
+
+    device_msg_destroy(&msg);
+}
+
+int main(void) {
+    test_deserialize_clipboard();
+    return 0;
+}