[FEAT] File association between .nxrp and SuperLemmix Player

[WillLem]

I originally requested this as a NeoLemmix feature back in Dec 2020, and have finally got around to figuring it out!

If Windows file association is set for .nxrp files, clicking on them now loads the level into the SLX player, opens straight to preview screen, and loads the replay into memory. Clicking into the level from here then plays the replay. The presence of >1 level with the same ID is accounted for with a selectable list dialog, and it's possible to cancel loading from here if the user is unsure which level to select.

Here's a video to demonstrate the feature. Super chuffed with how this one has turned out.

Some concerns for further development:

Cancelling exits to Menu, but it's also possible to exit SLX entirely - it's difficult to decide which is more preferable here. I suppose it depends entirely on use case, and possibly on this:

Currently, opening multiple replay files in this manner will open multiple instances of the SLX player, each with the relevant level & replay loaded. I personally think this is OK, and I'm not 100% sure if there's a way to load directly into an currently-open instance. Might investigate this further if people think it would be worth it.

This will definitely be present in 2.7 (due end of April). Implemented in Commit 1aa6df537