Update pick functions

[ffreyer]

This pr ...

• formally adds pick(scene, rect), which was already in GLMakie, but not connected and I believe not working.
• adds pick_closest(scene, screen, xy, range) as fallback/implementation for pick(scene, xy, range).
• adds pick_sorted(scene, screen, xy, range) to get all plots and indices in a given range of xy, sorted by distance. Required for Data inspector #677.

Requires JuliaPlots/GLMakie.jl#177.

I checked the following with a small scatter plot

• pick(scene, pos)
• pick(scene, pos, range) with and without overloads from GLMakie
• pick(scene, rect)
• pick_sorted(scene, pos, range) with and without overloads from GLMakie

This removes what pick(screen, rect) for unique(pick(scene, rect)) so it's technically a breaking change. But I think that function was broken anyway and didn't follow the usual syntax.

[ffreyer]

With this backends need to implement:

• pick(scene, screen, xy) -> (plot, idx) (like before)
• pick(scene, screen, rect) -> Matrix{Tuple{AbstractPlot, Int}} (sort of new)

which is somewhat simpler than before.

They can also implement

• pick_closest(scene, screen, xy, range) -> (plot, idx)
• pick_sorted(scene, screen, xy, range) -> distance sorted Vector{Tuple{AbstractPlot, Int}}

if those can be done more efficiently