Add a waring about unsupported save files

[hexagonrecursion]

• Fixes Crash when loading a level after changing AI code #1623
• Also addresses some concerns I have about a number of edge cases where public classes or public functions may cause a crash when loading a save if there were changes to the CBot API (even if the changes do not break backward compatibility).

In #1668 (comment) @Emxx52 told me "loading old save files is generally unsupported".

1. Loading an unsuported crashsave or quicksave now behaves as if there is not save
2. The -loadsave option also does not recognise a save as valid unless its version is supported
3. Loading an unsupported save via the save list asks the player to confirm

Note to self: after this is merged I should implement a fix for:

• Underwater alien worms #1624
• Terranova 9.2 - mission impossible? #935

[hexagonrecursion]

• Blocked: waiting for Convert even more paths to std::filesystem::path #1781 to land due to merge conflicts