GUI elements example screenshots

[AMDphreak]

I intend to update some of the documentation with screenshots.

Notably:

• Pane
• Panel
• message-box

Additional:

• message-box/custom
• get-directory
• get-file
• put-file (i.e. what's the difference from get-file)
• message+check-box
• message+check-box/custom
• get-text-from-user
• get-choices-from-user
• get-color-from-user
• get-font-from-user
• get-ps-from-user
• get-page-setup-from-user
• will add more to this list as I encounter.

Docs: https://docs.racket-lang.org/gui/Windowing_Functions.html

Assign to me if possible.

[AMDphreak]

@samth Do you know how the screenshots in this section of the docs were created? https://docs.racket-lang.org/gui/Widget_Gallery.html

[capfredf]

@AMDphreak you can take a look at https://github.com/racket/gui/blob/master/gui-doc/scribblings/gui/widget-gallery.scrbl#L14