Ported 'dents' effect from old version of Paint.NET

[Lehonti]

Here is the code of the original effect: https://github.com/rivy/OpenPDN/blob/cca476b0df2a2f70996e6b9486ec45327631568c/src/Effects/DentsEffect.cs

The icon is from that old version of Paint.NET, too: https://github.com/rivy/OpenPDN/blob/cca476b0df2a2f70996e6b9486ec45327631568c/src/Resources/Icons/DentsEffectIcon.png

I also refactored the methods for Perlin noise calculations (used by CloudsEffect, too) into their own class.

Tests fail because WarpEffect depends on PintaCore being initialized, so I commented them out the tests. We could either:

• Keep the commented-out tests in the codebase, and uncomment them in a future commit.
• Discard the last two commits (the ones that have the word 'revert' twice), so nothing would be added to the EffectsTest.cs file. It would still be fine because we have this PR as reference for when refactoring happens, and it would be less work to add the tests then.

[cameronwhite]

I think I'm good to keep the tests there but disabled for now until the PintaCore dependency can be fixed 👍