Replace fixed-point with exact notation in Matrix and Compound vis

[axelboc]

As discussed in #1646 (comment).

Keeping the issue open until I address the formatting of numbers in the visualizations' tooltips.

This should also address the user need of #1742 in a more usable way than the solution proposed of adding a control to the toolbar to increase the fixed-point precision.

While I'm at it, I bring the formatting of complex numbers in line with the formatting of other numbers. We were so far using formatters with less precision to save space in the Matrix vis, but I think consistency should prime.

Here is a more precise list of user-facing changes:

1. < Scalar > Display complex numbers with exact precision
2. < Scalar > Always show both real and imaginary parts of complex numbers to avoid confusing complex numbers without imaginary parts with real numbers — this applies also when inspecting values of complex attributes in the metadata viewer
3. < Matrix, Compound > Replace Fixed-point notation with Exact notation to allow inspecting and copying exact values
4. < Matrix, Compound > Inherit integer and/or float formatting for complex numbers in Auto and Scientific notations (depending on dtype of real and imaginary parts) — concretely, complex numbers are now displayed with more precision regardless of the notation.

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[axelboc]

/approve

[loichuder]

Again, excellent implementation 👏