Implement clipboard paste

Paste computer clipboard to the device on Ctrl+v.

The other direction (pasting the device clipboard to the computer) is
not implemented. It would require a communication channel from the
device to the computer, other than the socket used by the video stream.

app/src/main.c

@@ -82,6 +82,9 @@ static void usage(const char *arg0) {
         "    Right-click\n"
         "        turn screen on\n"
         "\n"
+        "    Ctrl+v\n"
+        "        paste computer clipboard to device\n"
+        "\n"
         "    Ctrl+i\n"
         "        enable/disable FPS counter (print frames/second in logs)\n"
         "\n",