This is the first step toward bundling all assets related to a save game
into a single item. That makes it easier to avoid clobbering "temporary"
assets from other games like the state.json, but also makes it easier
for players to file bug reports, since there's only a single asset to
upload.
This is only the first step because so far it only includes the various
save files: start of turn, end of last turn before results processing,
and "latest" (the game saved explicitly by the player).
Adds a do not show again checkbox to the empty install dir warning popup on lib start. This only appears when the install dir is empty to prevent mission scripting replacement.
Also adds a warning message to the log when the dir is empty. Will help to identify this in bug reports.
This will allow expert users to disable the automatic MissionScripting.lua replacement. There are many warnings and errors which have to be ignored to achieve this because DCS Liberation will not work with unmodified MissionScripting.lua
