[ShipsLogControl] Fix update failure with duplicate text

[csibbitt]

• Add a sequence # to log messages
• Store seq in GuiAdvancedScrollText along with rest of log data
• Compare seq instead of text when sync'ing ScrollText to log
• Fixes Issue #1947

I'm hoping this sequence # will also come in useful when I look at optimizing the network sync of the ship's log.

[daid]

Looks good enough for now. On the ECS branch this will be handled a bit differently. But, we're not there yet :)