Set compat of new instances based on Unity updates and alert user

[HebaruSan]

Motivation

One of the top user support requests at the moment is still how to make more mods show up as installable, with the answer still being to set compatible versions in the settings to work around still-missing compatibility metadata updates from mod authors:

Cause

New users have no concept of the history of game updates and mod updates and how they interacted; they see a more or less static landscape, and it doesn't make sense to them that they would have to change settings to install anything.

Meanwhile, a great number of mods that still work in the latest version were abandoned long before it was released, and so their compatibility still only goes up to whatever was the latest game version at that time.

Changes

• Now IGame.DefaultCompatibleVersions(GameVersion) defines which additional versions should be considered compatible for a given installed game version by default (user can still edit)
  • For KSP1, this is implemented as returning the major+minor versions between the most recent significantly compatibility-breaking Unity upgrade and the installed verson, inclusive. For KSP 1.12.5, the default is 1.8–1.12.
  • For KSP2, this is implemented as covering all released game versions (0.1 and 0.2), because version compatibility never really became a thing for that game
  • A few new tests exercise the above behaviors
  • When a game instance is loaded, if it has no compatible versions set:
    • The above default range is copied into the settings
    • The compatible game versions popup appears with a message about the defaulting that just happened, so users can know about it and take action if they don't like it
      
• The extra OK-only popup that announced an updated game version is now replaced by a top-message as for the above feature (even if no one ever sees it again)

This should help new users to find the mods they're looking for with a lower support burden, and expert users who prefer other settings can select them in the pop-up.