Enable nullability for Cell and co. and clean up

[YoshiRulz]

Starting on #3869 (comment). This builds upon that PR and the subsequent commits through to d67b1c1, mainly 4f144c7.

[Morilli]

I do wonder, is this even the correct direction to go? As in, you made Column and RowIndex explicitly nullable, but could we make them explicitly not-null? What's even the realistic usecases for constructing a (non-null) Cell object with either null RowIndex or null Column?

If either of those (or both) could be non-null, it would most likely simplify the code.

[YoshiRulz]

I agree, but in practice they both are set to null sometimes, for example:

BizHawk/src/BizHawk.Client.EmuHawk/CustomControls/InputRoll/InputRoll.cs

Lines 1093 to 1097 in 2206571

	CurrentCell = new Cell
	{
	Column = null,
	RowIndex = null
	};

[Morilli]

Yeah I've seen those cases in the code, but this one for example could just be replaced with CurrentCell = null, right?

[YoshiRulz]

¯\_(ツ)_/¯

[YoshiRulz]

I think this should be merged, and if the simplification discussed above is possible, it can be done later.