[SuperLemmix] About .sxlv level format

[WillLem]

SuperLemmix now has its own dedicated level format (.sxlv) as of the next version (3.0).

This is internally identical to .nxlv, so levels can easily be converted between the two formats (in most cases by simply changing the file extension).

Meanwhile, .nxlv will continue to be supported by both SuperLemmix and the SuperLemmix Editor.

The reasons why I've decided to go ahead with this are these:

1) We can now much more easily determine what is a SuperLemmix level and what is a NeoLemmix level (with the latter always being compatible with both).

2) From now on, if a level made in the SuperLemmix Editor contains SuperLemmix-specific features (such as the Ballooner skill, Collectibles, Superlemming mode, etc), it cannot be saved to .nxlv. This is to help authors choose the correct format when saving a level (they may later decide to change the extension manually if they wish to do so for whatever reason).

3) For users with both SuperLemmix and NeoLemmix installed, it will now be possible to file-associate levels with either the SuperLemmix Editor or the NeoLemmix Editor (which will soon be released as its own update). This will be much more useful when editing levels for a specific engine.

Finally, the SuperLemmix Editor can cleanse a full pack of levels with either .sxlv, .nxlv, or the original extension as the output format, so authors of existing packs can quickly convert their levels in the event that they contain SuperLemmix-specific features. Note that levels containing SuperLemmix-specific features will always be saved to .sxlv during a cleanse.

I hope that explains everything. Any questions, please ask!