DataInspector callback attributes

[ffreyer]

Description

This adds a lookup for callback attributes in DataInspector. Specifically:

• inspector_label = (plot, index, position) -> label which should return a string to be displayed when hovering a plot. Depending on the plot type index may also be a pair of indices (heatmap, image) and position can be a Point, bounding box (mesh) or pair of points (arrows).
• inspector_hover = (inspector, plot, index[, source_plot]) -> ... which takes over the full tooltip setup, i.e. replaces show_data
• inspector_clear = (inspector, plot) -> ... which triggers when no plot is hovered and can be used for extra cleanup

None of these need to be implemented. They change, replace or extend default behavior.

Closes #1759, #1902. For example:

fig = Figure()
ax = Axis(fig[1, 1])
scatter!(ax, rand(10), inspector_label = (self, i, p) -> "$p")
lines!(ax, rand(10), inspector_label = (self, i, p) -> "$p")
DataInspector(fig)
fig

This is branched off of #2095, so that should be merged first.

Type of change

• New feature (non-breaking change which adds functionality)

Checklist

• [ ] Added an entry in NEWS.md (for new features and breaking changes)
• [ ] Added or changed relevant sections in the documentation
• [ ] Added unit tests for new algorithms, conversion methods, etc.
• [ ] Added reference image tests for new plotting functions, recipes, visual options, etc.
• [ ] Check that this doesn't break with CairoMakie and WGLMakie