Setting v/h- expand to true

[yakir12]

There is a discussion about setting the vexpand and hexpand properties of canvas to true since there is some expectation that the canvas would expand with its parent window (see https://discourse.julialang.org/t/allow-for-auto-scaling-of-image-in-gtk4/111624/2).

I assume that that can be easily achieved with adding ; hexpand = true, vexpand = true to:

GtkObservables.jl/src/graphics_interaction.jl

Lines 327 to 328 in b18ac4d

	canvas(::Type{U}=DeviceUnit, w::Integer=-1, h::Integer=-1; init_back=false, modifier_ref=nothing) where {U<:CairoUnit} = Canvas{U}(w, h; init_back=init_back, modifier_ref=modifier_ref)
	canvas(w::Integer, h::Integer; init_back=false, modifier_ref=nothing) = canvas(DeviceUnit, w, h; init_back=init_back, modifier_ref=modifier_ref)

Or we could simply document that somewhere....

I'm open to PR either option.

[jwahlstrand]

I agree this should be the default -- there's a PR in #66