config_["hide-vacant"].asBool() was called from the river status listeners.
When the option is given as a string ("true") jsoncpp's asBool() throws
"Value is not convertible to bool", and that exception unwinding through
libwayland's C dispatch aborts the process. Parse it once in the constructor
into a bool member (accepting the string form) and read the cached value.
Fixes#4078
Before this, vacant tags would show with `hide-vacant` set on initial
startup, because we receive initial tag events from River before we show
the bar. In that case, we won't call `set_visible(false)` on the
respective buttons because they're not shown yet. This registers the
output status listener after we show the bar so we won't miss any
events.
Alex