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>

README.md

@@ -287,6 +287,7 @@ you are interested, see [issue 14].
  | turn screen on                         | _Right-click²_                |
  | expand notification panel              | `Ctrl`+`n`                    |
  | collapse notification panel            | `Ctrl`+`Shift`+`n`            |
+ | copy device clipboard to computer      | `Ctrl`+`c`                    |
  | paste computer clipboard to device     | `Ctrl`+`v`                    |
  | enable/disable FPS counter (on stdout) | `Ctrl`+`i`                    |