NEXXT 3.7.2 released - minor fix

This update isn't essential for most, but still. 

On the metasprite tab, there used to be a niche mouse action called "mirror move", which moved sprites apart or together,
occasionally useful for animating certain particle effects.
Since 3.7.0, this got disabled from more essential mouse actions taking precedence on the same key binding. 

3.7.2 re-introduces "mirror-move" by putting it on the same +alt modifier as the almost equally niche "swirl move".
You choose which move action applies by clicking the [...] button on the metasprite tab and navigate to "+alt means..." on the following popup menu.