Add subtitles to Axis

[jkrumbiegel]

Description

Adds subtitle functionality to Axis which only has titles so far.
Can be used for something like this:

f = Figure()
ax = Axis(f[1, 1],
        title = "Trigonometric functions",
        subtitle = "Sine, cosine and tangens are the most used functions.",
        titlealign = :left,
    )
lines!(ax, -pi/3..pi/3, sin, label = "sin")
lines!(ax, -pi/3..pi/3, cos, label = "cos")
lines!(ax, -pi/3..pi/3, tan, label = "tan")
Legend(f[1, 2], ax)
f

Type of change

Delete options that do not apply:

• 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 reference image tests for new plotting functions, recipes, visual options, etc.

[MakieBot]

Compile Times benchmark

Note, that these numbers may fluctuate on the CI servers, so take them with a grain of salt.

using time

master:  12.47 < 12.66 > 12.92, 0.14+-
pr:      12.27 < 12.58 > 12.81, 0.17+-
speedup: 0.99 < 1.00 > 1.05, 0.02+-
median:  -0.64% => invariant

This PR does not change the using time.

ttfp time

master   31.23 < 31.87 > 32.69, 0.56+-
pr       32.14 < 32.60 > 32.94, 0.32+-
speedup: 0.97 < 0.97 > 1.01, 0.02+-
median:  +2.29% => invariant

This PR does not change the ttfp time.