Implement debuglog command

Add debuglog command that takes toggle|on|off. Add get_debug_logging()
to be able to toggle. Make t/187-commands-parser.t expect 'debuglog'.
Document the debuglog command in userguide.
bugfix: Resize and center a scratchpad even when a criteria is used.
actively delete _NET_WORKAREA on startup

fixes #1038
i3-dmenu-desktop: honor Path= key (Thanks Alexander)

fixes #1041