Feature: Implement keypress and mouse_event default handlers

[penguinolog]

• Widget class to have default standard handlers
• Produce debug log for non-selectable widgets received events
• Produce WidgetWarning if __init__ of base class not called
• Produce deprecation warnings if no subclassed Widget

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)

[penguinolog]

This PR should be used instead of #718 : do not break existing code

[coveralls]

Pull Request Test Coverage Report for Build 7388485737

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+1.1%) to 72.118%

---

Totals
Change from base Build 7337088868:	1.1%
Covered Lines:	7770
Relevant Lines:	10897

---

💛 - Coveralls

[wardi]

should also update docs if subclassing Widget will be required

[penguinolog]

should also update docs if subclassing Widget will be required

https://github.com/urwid/urwid/blob/master/docs/manual/widgets.rst?plain=1#L656

It is already in documentation for the last 11 years

[penguinolog]

Validated pudb: fully compatible. With all enabled warnings:

:1: ResourceWarning: unclosed file <_io.TextIOWrapper name='tst.py' mode='r' encoding='UTF-8'>

(Source is not urwid side)

[penguinolog]

Validated nomadnet, s-tui and mitmproxy: no failures