Fix scenario messages being displayed only after Logbook is hidden.

Scenario messages are now stored into logbook._messages_to_display, and are only displayed on logbook.hide(). This usually works because when scenario messages are received, the logbook is shown, and to see the messages, you need to hide the logbook.
The problem is that when loading the game, scenario messages are only shown after showing and hiding the logbook. To fix this, I put the code used for showing messages into a function and called this function when the logbook is loaded.
Fixes #1790