Remove GLMakie.Screen() requirement for opening a new window.

[nathanrboyer]

This was suggested in the original pull request but not implemented for some reason.
#1771 (comment)

Will you consider having display(fig1); display(fig2) open two windows without the need to remember/write display(GLMakie.Screen(), figure_or_scene)? It seems to me that a figure should always be associated with a separate window.

If not automatically every time, then maybe something easier to remember like a keyword argument display(fig2, new_window=true)?

(I would like to see the AlgebraOfGraphics draw function automatically open new windows too. I don't know if that would happen implicitly with this change or not.)

[jkrumbiegel]

This seems like something that should be a settable preference to me. Some people will prefer a new window every time, some won't because it clutters their workspace. I also don't think display(GLMakie.Screen(), figure_or_scene) is easy to remember.

[dav2017]

You can also create your own shortcut command, eg.:

dispnew(figure) = display(GLMakie.Screen(), figure)

[bc0n]

I was glad to discover just now that Makie can display multiple windows, which is to say that the single-window default behavior is not what I expected. For instance, Matlab uses the figure handle to control where data is plotted, drawing the window at the first figure() or plot() command without an explicit display().
If display() has different arguments it should clearly open multiple windows, while calling display multiple times on the same figure is redundant or an indirect attempt to animate the window.