Previously we used a `CellValue` class to hold the data for a cell,
and called a function to fill it whenever necessary. This is an
unnecessary complication when we have virtual generic arrays
and most data is already easily accessible that way anyway.
This patch removes `CellValue` and uses `fn::GVArray` to provide
access to data instead.
In the future, if rows have different types within a single column,
we can use a `GVArray` of `blender::Any` to interface with the drawing.
Along with that, the use of virtual arrays made it easy to do a
few other cleanups:
- Use selection domain interpolations from rB5841f8656d95
for the mesh selection filter.
- Change the row filter to only calculate for necessary indices.
Differential Revision: https://developer.blender.org/D13478
474adc6f88