[BUG] plot_title affects subplot indexing because implemented as additional subplot which just emulating plot title

[EldarAgalarov]

This code creates plot with 2 subplots with indexes 1 and 2 and length = 2:

using Plots

Plots.reset_defaults()
default(
    background_color = :darkgray,
    background_color_inside = :lightgray
)

p1 = plot(1:100, rand(100))
p2 = plot(1:100, rand(100))
p = plot(p1, p2, layout = grid(2, 1))

p_len = length(p) # 2 subplots
sp1 = p[1] # subplot 1
sp2 = p[2] # subplot 2

When adding plot_title attribute to parent plot then indexing becoming broken because plot_title is just additional subplot that added at the top of the layout:

using Plots

Plots.reset_defaults()
default(
    background_color = :darkgray,
    background_color_inside = :lightgray
)

p1 = plot(1:100, rand(100))
p2 = plot(1:100, rand(100))
p = plot(p1, p2, layout = grid(2, 1), plot_title = "plot_title")

p_len = length(p) # 3 subplots (added additional subplot that contains title)
pt = p[1] # plot title
sp1 = p[2] # subplot 1 (index changed!)
sp2 = p[3] # subplot 2 (index changed!)

This is definitely a bug, because it breaks existing code that relies on accessing subplots based on indices.
Adding plot_title is just cosmetic change, not structural and shouldn't affect on subplot indexing.

Related bug that affects background coloring of plot_title area.