Allow signal handling interoperability with raw display

[AnonymouX47]

Checklist

• I've ensured that similar functionality has not already been implemented
• I've ensured that similar functionality has not earlier been proposed and declined
• I've branched off the master or python-dual-support branch
• I've merged fresh upstream into my branch recently
• I've ran tox successfully in local environment
• I've included docstrings and/or documentation and/or examples for my code (if this is a new feature)

Description:

Currently, it's either urwid's handler or another... except an/some aggregate handler(s) (which calls urwid's and the other(s)) is/are installed by the program using the different libraries, which in certain cases might either be difficult to trace where/when each library installs its handler or be implementation detail which could change without notice.

This changes ensure any previously installed signal handlers are stored, restored and invoked (by raw_display.Screen's signal handlers).
This prevents breaking functionality of other libraries which install signal handlers and are unaware of urwid.

I encountered this issue while developing a library where SIGWINCH would be really useful.

Sample test code:

import os
import signal
import urwid

def sigwinch_handler(signum, frame):
    text.set_text("x".join(map(str, os.get_terminal_size())))

signal.signal(signal.SIGWINCH, sigwinch_handler)

text = urwid.Text("x".join(map(str, os.get_terminal_size())), "center")
try:
    urwid.MainLoop(urwid.Filler(text)).run()
except KeyboardInterrupt:
    pass