Implement computer-to-device clipboard copy

It was already possible to _paste_ (with Ctrl+v) the content of the
computer clipboard on the device. Technically, it injects a sequence of
events to generate the text.

Add a new feature (Ctrl+Shift+v) to copy to the device clipboard
instead, without injecting the content. Contrary to events injection,
this preserves the UTF-8 content exactly, so the text is not broken by
special characters.

<https://github.com/Genymobile/scrcpy/issues/413>

app/src/main.c

@@ -143,6 +143,9 @@ static void usage(const char *arg0) {
         "    Ctrl+v\n"
         "        paste computer clipboard to device\n"
         "\n"
+        "    Ctrl+Shift+v\n"
+        "        copy computer clipboard to device\n"
+        "\n"
         "    Ctrl+i\n"
         "        enable/disable FPS counter (print frames/second in logs)\n"
         "\n"