skip annotations for plot title subplot

[InfiniteChai]

Description

When applying annotations to a plot which has used plot_title the behaviour is inconsistent. Applying it as part of the initial plot will not apply the annotation to the title sub-plot but doing it separately will.

scatter([4], [4], plot_title = "x", xlims = (0, 10), ylims = (0, 10), annotation=(4,4,"4"))

versus

scatter([4], [4], plot_title = "x", xlims = (0, 10), ylims = (0, 10))
annotate!(4, 4, "4")

It seems sensible to never have annotations applied to the plot title sub-plot, so updating to ignore these property updates and get consistent behaviour. This fixes #5096

[t-bltg]

LGTM