disable a GC test when running CGC
racket/gui cocoa: avoid focus-setting loop
mrlib/terminal: create embedded terminal in a container sequence
racket/gui cocoa: fix swapping of buttons with the 'border style

Register a button as a default button every time that it is shown,
so that multiple buttons in a frame can have the 'border style
(as long as only one is shown at a time).
raco pkg: report when downloading