Added timestamps in Chatlog

It was a little odd that timestamps checkbox in Preferences affected
only regular chat messages, but not the chatlog, so I decided to add
timestamps into the chatlog.

I hate to use `const_cast` to add a timestamp to a received message, so
if someone knows a better solution, don't hesitate to mention it.
Modified timestamps

Modified the previous commit after discussing it with Soliton. We agreed
on sending timestamps to other players and using them [received timestamps]
in displaying time of chat messages, with fallback on using local time if
received message doesn't contain timestamp (old version of wesnoth).

This fixes several issues:
1) When viewing replays the time would be the one of the actual game
insted of the time of viewing the replay;
2) Previously, when opening some dialog while players send messages and
then closing it — chat messages would have timestamp of when the dialog
windows was closed, not the time when messages were sent.

This also introduces one new issue:
1) If player has time set up incorrectly, the timestamp that other players
will recieve will show incorrect time.
For example, when instead of setting to the right timezone player just
adjusted system time by adding or subtracting hours with incorrect
timezone selected.
Refactoring

Renamed undescored variable so it won't get taken for class variable by
mistake.
Merge pull request #4 from nurupo/master

Added timestamps in Chatlog