Generate an icon sprite map with new sprites and 2x retina graphics

Summary:
  - The filesystem is now the authority for which sprites are available. If you add new icons, the generation process will pick them up.
  - I broke out icon generation and added retina support. App icon generation still uses the old method.
  - Update ActionList and RemarkupControl to use the new sheet.
  - Use white icons on hover.
  - Also fixed a couple of minor issues with some stuff in Firefox/Chrome.

Test Plan:
{F25750}

{F25751}

{F25752}

Reviewers: chad

Reviewed By: chad

CC: aran

Maniphest Tasks: T2013

Differential Revision: https://secure.phabricator.com/D4027

scripts/celerity/generate_sprites.php

@@ -27,7 +27,8 @@ if (!$srcroot) {
     "You must specify a source directory with '--source'.");
 }
 
-$webroot = dirname(phutil_get_library_root('phabricator')).'/webroot/rsrc';
+$root = dirname(phutil_get_library_root('phabricator'));
+$webroot = $root.'/webroot/rsrc';
 $webroot = Filesystem::readablePath($webroot);
 
 function glx($x) {
@@ -192,10 +193,12 @@ $sheet->generateCSS($webroot.'/css/autosprite.css');
 
 $generator = new CeleritySpriteGenerator();
 
-$icon_sheet = $generator->buildIconSheet();
-$icon_sheet->setScales(array(1, 2));
-$icon_sheet->generateImage($webroot.'/image/sprite-icon.png', 1);
-$icon_sheet->generateImage($webroot.'/image/sprite-icon-X2.png', 2);
-$icon_sheet->generateCSS($webroot.'/css/sprite-icon.css');
+$generator
+  ->buildIconSheet()
+  ->setScales(array(1, 2))
+  ->generateImage($webroot.'/image/sprite-icon.png', 1)
+  ->generateImage($webroot.'/image/sprite-icon-X2.png', 2)
+  ->generateCSS($webroot.'/css/sprite-icon.css')
+  ->generateManifest($root.'/resources/sprite/manifest/icon.json');
 
 echo "Done.\n";