Add subtle borders to widgets (#167773)

* Add border to quick pick to match menus

* Add more borders and border radius to widgets

* More border updates

* Move color defaults to themes

* Address feedback

extensions/theme-defaults/themes/dark_vs.json

@@ -24,6 +24,7 @@
 		"tab.lastPinnedBorder": "#ccc3",
 		"list.activeSelectionIconForeground": "#FFF",
 		"terminal.inactiveSelectionBackground": "#3A3D41",
+		"widget.border": "#303031",
 	},
 	"tokenColors": [
 		{