Add JSON support to save game

[royfalk]

• Game will only save as JSON.
• Game will try to load JSON and then legacy.

Also:

• Remove duplicate parameter system in ParseSaveGame.
• Remove function getCurrentSaveGame.
• Remove unused API getCurrentSaveGame. Is not used in either VS or privateer. Delete basecomputer

Code Changes:

• Have the PR Validation Tests been run? See https://github.com/vegastrike/Vega-Strike-Engine-Source/wiki/Pull-Request-Validation
• This is a documentation change only

Issues:

• Save game is about 20% larger. 10MB vs. 12MB, double the size 24MB vs 10MB. However, we can improve this by either zipping the file or figuring out what is being saved. I suspect it is saving the universe state and that could be done once per installation.
• The save game mechanism is unclear to me. I don't mean the format of the file. That's almost clear. I mean what the game saves and why. This requires more research.

[royfalk]

The save game saves two types of mission data - mission data and mission string data.
It appears mission data is a legacy format we can drop it.

"This variant loads it from float data, converting to character data. It's a legacy variant, highly and unpleasantly convoluted and sub-performant, but we must support it to be compatible with old savegames"

I checked and the only place in it's used in assets is in New_Game. The mission is unit_to_dock_with and the value is ascii of Atlantis in floats.

[royfalk]

And yet, somehow it is used for storing news. Somehow, it is accessing the variable and putting data there. Hmm.

[royfalk]

@evertvorster can you upload a save game with more content. I mean a save game after you've played a few hours at least. Killed enemies. Did missions. etc.