Add Ctrl+X for roomname translator mode

This is a single block of code so it was easy to split from the
foundation commit.

This commit is part of rewritten history of the localization branch.
The original (unsquashed) commit history can be found here:
https://github.com/Dav999-v/VVVVVV/tree/localization-orig

desktop_version/src/KeyPoll.cpp

@@ -196,6 +196,14 @@ void KeyPoll::Poll(void)
                         VVV_free(text);
                     }
                 }
+                else if (    evt.key.keysym.sym == SDLK_x &&
+                        keymap[SDLK_LCTRL]    )
+                {
+                    if (SDL_SetClipboardText(keybuffer.c_str()) == 0)
+                    {
+                        keybuffer = "";
+                    }
+                }
             }
             break;
         }