Add momentary shortcuts to toggle grid/distance snap

[peppy]

Matching osu!stable. I use these quite a lot while mapping and I'm sure others do as well.

Hold Shift = invert grid snap
Hold Alt = invert distance snap

• Depends on Allow both distance snap and grid snap to be applied at the same time #20914

[peppy]

I've fixed all the known edge cases I could find.

Note that to make this work well, I limited the momentary toggles so only one can be utilised at once. If not for this, a case like the following could occur, requiring a stack of application history (and even then, it could go wrong if the mouse is used to toggle via a button):

• Distance snap is enabled
• Momentary enable grid snap (forces distance snap off)
• Momentary enable distance snap
• Let go on opposite order of momentary presses
• Distance snap is now off

I want to get this as close as possible without getting too complex, and I think what is on this branch should work for now.

[bdach]

Found one more edge case that wasn't mentioned anywhere above, osu!-specific:

1. Turn on permanent grid snap by clicking the ternary button w/ mouse
2. Press and release Alt to momentarily toggle distance snap
3. After releasing Alt, grid snap is now permanently off

2022-10-21.22-51-49.mp4

Works the other way round too, obviously.

Unsure if you're aware of this, so holding off from doing anything further with this pull yet. I'm not hugely fussed, but it feels a little bad that this happens and I don't think it's not fixable with a reasonable amount of effort.

[peppy]

I guess that's the same case I pointed out but reproduced much simpler. It's unfortunate and I'd like to fix it... but as you say it's going to take considerable thought.

[peppy]

Should work better now (with the dependent PR).

[bdach]

Implementation looks good, one slight problem, however...