LivestatusBackend: just an empty backend
Filter: allow to list all filtered columns

This is important for backends that need to know which additional columns
to fetch.

refs #4787
Filters: try to give meaningful results for...

...missing columns

refs #4787
Merge remote-tracking branch 'origin/master' into feature/livestatus-playground-4787
Widget\Tabs: use icon font
LiveStatus\StatusQuery: should be a Livestatus\Query
Livestatus\Query: extend SimpleQuery, not abstract
Livestatus\Query: remove SimpleQuery tasks
Livestatus\Query: remove obsolete properties
Livestatus\Query: temporarily disable count cache
Livestatus\Query: split string cast implementation

This way we can use an exception-throwing variant internally while not
breaking __toString
Livestatus\StatusQuery: add terribly setMode method
Livestatus\Connection: temporarily hardcode count
Livestatus\Query: parse an array structure
Livestatus\Query: fetch csv, not json
Livestatus\Query: add a lot of filter voodoo
Livestatus\Connection: add fetchRow / fetchPairs

Temporary ugly and slow implementation
Livestatus\Query: use trigger_error in __toString
Livestatus\Query: attempt to separate column handling
Livestatus\Query: start using new filter methods