Implement device-to-computer clipboard copy

On Ctrl+C:
 - the client sends a GET_CLIPBOARD command to the device;
 - the device retrieve its current clipboard text and sends it in a
   GET_CLIPBOARD device event;
 - the client sets this text as the system clipboard text, so that it
   can be pasted in another application.

Fixes <https://github.com/Genymobile/scrcpy/issues/145>

app/src/main.c

@@ -137,6 +137,9 @@ static void usage(const char *arg0) {
         "    Ctrl+Shift+n\n"
         "       collapse notification panel\n"
         "\n"
+        "    Ctrl+c\n"
+        "        copy device clipboard to computer\n"
+        "\n"
         "    Ctrl+v\n"
         "        paste computer clipboard to device\n"
         "\n"