Wait / poll / until condition

[tony]

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• Required for: https://github.com/tmux-python/tmuxp/issues/96
• Related to https://github.com/tmux-python/libtmux/pull/372/files#r874769282, #368

Down the road, I'm happy baking waiting / polling with a timeout into libtmux itself - if the API looks pleasing.

Example of what I mean (though the APIs look archaic):

• Selenium: https://selenium-python.readthedocs.io/waits.html
• Splinter's wait_time: https://splinter.readthedocs.io/en/latest/api/driver-and-element-api.html
• boto3 waiter's: https://boto3.amazonaws.com/v1/documentation/api/latest/guide/clients.html#waiters
• requests timeouts: https://docs.python-requests.org/en/latest/user/advanced/#timeouts