Consider replacing sealed/unsealed messages with SystemLogMessage

[quisquous]

See #4199.

Timeline syncs could be become something like:

-0.0 "--Reset--" sync / 00:0839::.*is no longer sealed/ window 100000 jump 0
+0.0 "--Reset--" sync / 29:[^:]*:7DE:/ window 100000 jump 0
-1000.0 "--sync--" sync / 00:0839::The Shell Mound will be sealed off/ window 10000,0
+1000.0 "--sync--" sync / 29:[^:]*:7DC:[^:]*:1008:/ window 10000,0

This is less readable (unfortunately), but has the advantage that it doesn't use chat lines (which could be messed up by dalamud plugins or turned off for privacy), and doesn't require translations. It would also require a little bit more tool work in make_timeline/test_timeline to emit these.

We also couldn't do this change until May when the Korean version gets to 6.x and bumps up to the newer ffxiv plugin version where this log line exists.