input, player: new command for mouse event

New command `mouse <x> <y> [<button> [single|double]]` is introduced.
This will update mouse position with given coordinate (`<x>`, `<y>`),
and additionally, send single-click or double-click event if `<button>`
is given.

input/cmd_list.c

@@ -186,6 +186,13 @@ const struct mp_cmd_def mp_cmds[] = {
   { MP_CMD_HOOK_ADD, "hook_add", { ARG_STRING, ARG_INT, ARG_INT } },
   { MP_CMD_HOOK_ACK, "hook_ack", { ARG_STRING } },
 
+  { MP_CMD_MOUSE, "mouse", {
+      ARG_INT, ARG_INT, // coordinate (x, y)
+      OARG_INT(-1),     // button number
+      OARG_CHOICE(0, ({"single", 0},
+                      {"double", 1})),
+  }},
+
   {0}
 };