screen: Add set_screen_resize() method.

Sets a callback to run on terminal resize. Implemented using OS
SIGWINCH signal, and thus won't work in a general case (e.g. over
a serial connection).

Also, picotui stores absolute coordinates for each widget, so, to
handle resizing, all dialogs, etc. should be recreated from scratch
with new size.

picotui/screen.py

@@ -1,4 +1,5 @@
 import os
+import signal
 
 from .defs import *
 
@@ -155,3 +156,7 @@ def screen_size(cls):
     @classmethod
     def set_screen_redraw(cls, handler):
         cls.screen_redraw = handler
+
+    @classmethod
+    def set_screen_resize(cls, handler):
+        signal.signal(signal.SIGWINCH, lambda sig, stk: handler(cls))