Add option to change label and label value fontsize in SligerGrid

[pmc4]

Description

Fixes #2377

• Adds labelsize attribute to SliderGrid block to change the fontsize of the slider labels.
• Adds valuesize attribute to SliderGrid block to change the fontsize of the slider label values.

Maybe valuesize is not the most appropiate name. Should valuelabelsize or something else be used instead?

For reference, this is how it looks like on the example of the docs:

I have exagerated the fontsize of both elements to perceive the difference. The code is the following:

using GLMakie


fig = Figure()

ax = Axis(fig[1, 1])

sg = SliderGrid(
    fig[1, 2],
    (label = "Voltage", range = 0:0.1:10, format = "{:.1f}V", startvalue = 5.3),
    (label = "Current", range = 0:0.1:20, format = "{:.1f}A", startvalue = 10.2),
    (label = "Resistance", range = 0:0.1:30, format = "{:.1f}Ω", startvalue = 15.9),
    width = 350,
    tellheight = false,
    labelsize = 8,
    valuesize = 30)

sliderobservables = [s.value for s in sg.sliders]
bars = lift(sliderobservables...) do slvalues...
    [slvalues...]
end

barplot!(ax, bars, color = [:yellow, :orange, :red])
ylims!(ax, 0, 30)

fig

Type of change

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

Checklist

• Added an entry in CHANGELOG.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.