Optionally use system locale for chat timestamp

Optionally use system locale for chat timestamp

Using QLocale::system().timeFormat(), check if the AM/PM designator
exists; if so, assume a 12-hour style timestamp, otherwise assume the
previous 24-hour style timestamp.

Generate a timestamp of either " hh:mm:ss" or " h:mm:ss ap" in order
to include seconds (QLocale::ShortFormat does not specify seconds).

Add new setting UseCustomTimestampFormat to switch between system
or user timestamp string.  Bump settings version minor to keep old
behavior for upgrades.