[PATCH] make "close" on viewport synchronous

[Qian Yun]

Currently, when doing "close" on a viewport, it simply
send a number (via socket) to the viewman process,
then return to the Spad REPL immediately, while the
view2D/view3D process is being closed asynchronously.

Thus causes the ugly hack "|waitForViewport|".

We can avoid this by making "close" synchronous:
now the "close" function wait for confirmation of
exit of view2D/view3D and resources cleanup.

- Qian