Revamp the terminal paste mechanism using toplevel callbacks.

I've removed the ad-hoc front-end bodgery in the Windows and GTK ports
to arrange for term_paste to be called at the right moments, and
instead, terminal.c itself deals with knowing when to send the next
chunk of pasted data using a combination of timers and the new
top-level callback mechanism.

As a happy side effect, it's now all in one place so I can actually
understand what it's doing! It turns out that what all that confusing
code was up to is: send a line of pasted data, and delay sending the
next line until either a CR or LF is returned from the server
(typically indicating that the pasted text has been received and
echoed) or 450ms elapse, whichever comes first.

[originally from svn r10020]

windows/window.c

@@ -877,8 +877,6 @@ int WINAPI WinMain(HINSTANCE inst, HINSTANCE prev, LPSTR cmdline, int show)
 
 	    if (!(IsWindow(logbox) && IsDialogMessage(logbox, &msg)))
 		DispatchMessage(&msg);
-	    /* Send the paste buffer if there's anything to send */
-	    term_paste(term);
 
 	    if (must_close_session)
 		close_session();