libdeng2|Config: Don't warn about missing version info in serialized data

The version information can be missing under some circumstances
(first run, etc.).
libdeng2|LogBuffer: Allow changing outputs without flushing

This allows forcing log entries to a different output at startup
without flushing existing entries to the previously configured
output.

Also, automatic flushing is enabled when enableFlushing() is called.
UI|Client|Log: Added Config variable to show/hide entry metadata

Previously entry metadata has been hardcoded to be visible in debug
builds, however this now allows toggling it dynamically.