Better GTK support

[Lazerbeak12345]

racket/gui has no support for builtin gtk icons (out of the box), and most widgets (such as text-field%) don't need to be using a canvas under the hood (though, this canvas is capable of exposure, and thus backwards incompatible - Perhaps it could fall back to canvas when a canvas is requested?).

[xgqt]

my 5 cents: support for system tray would be nice :D

[jackfirth]

This is more of a racket/gui issue than a rhombus issue since I'm sure there's a backward-compatible way to achieve this. Could someone with appropriate permissions transfer this issue to the racket/gui repo?