Add "Hold Off" mod (no long notes)

[Spooghetti420]

Hello, I have made an osu!mania mod which converts all held notes into single-tap notes, one at the start and one at the end. It has a customization which allows users to select how long a held note needs to be for an end note to be added, if at all.
I saw this mod concept while my friend was playing Quaver and thought it might be a fun addition to osu! too. I hope the code is of acceptable quality, though I wasn't sure how to write a test, so sorry for any inconveniences.
I also don't feel awfully confident about the icon that represents the mod, nor the descriptions/tooltips for customization, so, by all means, please change them as necessary.
Thanks

[frenzibyte]

Thanks for the contribution.

I have made a few notes on the code, in addition to CI results. It would be best to have basic test coverage for this mod (see all osu.Game.Rulesets.Mania.Tests/Mods/TestSceneManiaMod*.cs for example)

[bdach]

there will be code style inspections due to too many empty lines here - please run inspectcode locally to resolve

[Spooghetti420]

Thanks for the suggestions, everyone. I've implemented all of the improvements for now, except the test "suite" is still a little lacking :-) It took a good deal to get it running, so maybe I'll return to it to add some more tests tomorrow, though I still don't know what test ideas to try out.
On a separate note — apologies for once again screwing up the formatting. I ran dotnet format before committing, but I guess that doesn't correct every single issue. I could push a commit to fix this, but I'm unsure as to whether this is a good idea.

[Spooghetti420]

Just changed the "threshold" we had before to derive from the beat value of a note rather than from its time in ms, so now only notes that last over 1/2 a beat (by default) get an end note. (Is the way I implemented it a good way to get a hold note's note value, though?)

Like @peppy and @smoogipoo said, it might be too excessive to have this kind of control specifically over end notes, but I included them anyway to see whether this setting is any good. Currently there's a dropdown that allows 1/1, 1/2, 1/4, etc. to be set as the thresholds, though please scrutinize this feature if it's not really appropriate.
Likewise, I've written several more tests with this setting concept in mind, though I cannot see them being useful if we remove the dropdown. I cannot guarantee the code is up to standard, but I've done my best for now. Thanks to everyone for all the feedback once again :)

[smoogipoo]

I think this mod should be made incompatible with ManiaModInvert, and be grouped under the same button.

[peppy]

Agree with incompatible. Disagree with same button (they are functionally very different and should be top-level visible).

[Spooghetti420]

I also don't know about grouping under the same button, but I have gone and made incompatible with Invert. I also did wonder whether they should be incompatible from the beginning, but I wasn't sure enough to commit it.

[smoogipoo]

For future time travelers who may be interested in improving/adjusting this, my intentions regarding beat length weren't implemented as I suggested them to be. Rather than adding an ending note when the length exceeds some beat length value, my intention was to add a note every X beat length.

It basically results in the following difference:
Original map: https://user-images.githubusercontent.com/1329837/152097084-a4b3dda6-d9c6-440f-80dd-ae85e2d69eff.mp4
This PR: https://user-images.githubusercontent.com/1329837/152097070-9aea110e-480d-47f9-9495-c0a24de8ae85.mp4
This PR with END_NOTE_ALLOW_THRESHOLD = 1: https://user-images.githubusercontent.com/1329837/152097048-f51bf7bc-254b-4219-91ef-258b8b357ed5.mp4
My suggestion with adding notes every 1 beatlength: https://user-images.githubusercontent.com/1329837/152097105-b8280067-bfe6-4666-8e68-2f3fdcaa2bfa.mp4

This takes advantage of two facts:

• Typically holds (whether in o!m or in osu!) end slightly before the next note, either 1/4 or 1/2 beat away, which allows the next note to arrive on the 1/1 beat.
• Hold ends in mania are typically hitsound-less for this same reason, so this current implementation generates a bunch of dead jack notes.

[peppy]

Sounds better. Could notes not just be added at every tick?

[smoogipoo]

Haven't explored that/don't know if mania properly uses tick rate at all (because it's not a thing in osu-stable), but possibly.