consolidated icon css in ui-components package

packages/application/style/base.css

@@ -46,9 +46,6 @@ body {
 /* Sibling imports */
 @import './datagrid.css';
 @import './dockpanel.css';
-@import './icons.css';
-@import './iconshover.css';
-@import './iconsalt.css';
 @import './menus.css';
 @import './scrollbar.css';
 @import './tabs.css';

packages/theme-light-extension/style/urls.css

@@ -5,6 +5,8 @@
 
 /* TODO: finish cleaning up these cases of icon-as-css-background-image */
 :root {
+  /* may be unused */
+
   /*--jp-icon-bug: url('icons/md/bug.svg');*/
 
   /* blocked by need for stack/carousel */

packages/ui-components/style/base.css

@@ -6,6 +6,9 @@
 /* Sibling imports */
 @import './deprecated.css';
 @import './deprecatedExtra.css';
+@import './icons.css';
+@import './iconsalt.css';
+@import './iconshover.css';
 
 /* Override Blueprint's _reset.scss styles */
 html {