drag column separator to resize previous column

[midichef]

Closes #3039.

Some things of note in this PR: for every click, the final command in the status bar is no longer no-op but check-drag.
It removes what I think is the last use of @Sheet.command. I don't know if the addCommand() that I changed it to is sufficient. It seems to work, but I'm not sure why @Sheet.command was originally used:

visidata/visidata/settings.py

Lines 379 to 386 in 43e129e

	def _command(cls, binding, longname, helpstr, **kwargs):
	def decorator(func):
	funcname = longname.replace('-', '_')
	setattr(vd, funcname, func)
	cls.addCommand(binding, longname, f'vd.{funcname}(sheet)', helpstr, **kwargs)
	return decorator
	BaseSheet.command = classmethod(_command)