Show favicon for about:treestyletab-group tab correctly on Firefox 22 and later (#521)

Because URIs like "chrome://treestyletab/skin/group/group-icon#-moz-resolution=16,16" doesn't work, if the chrome URL is defined by "override" directive.

chrome.manifest

@@ -30,8 +30,8 @@ locale	treestyletab	da-DK	locale/da-DK/treestyletab/
 
 skin	treestyletab	classic/1.0	skin/classic/treestyletab/
 
-override chrome://treestyletab/skin/group/group-icon	chrome://treestyletab/skin/group/group-fx3.png
+skin	treestyletab-group	classic/1.0	skin/classic/treestyletab-group-basic/
-override chrome://treestyletab/skin/group/group-icon	chrome://treestyletab/skin/group/group-fx3-aero.png	os=WINNT osversion>=6
+skin	treestyletab-group	classic/1.0	skin/classic/treestyletab-group-aero/	os=WINNT osversion>=6
 
 override chrome://treestyletab/skin/platform-base.css	chrome://treestyletab/skin/dummy.css	os=Darwin
 override chrome://treestyletab/skin/platform-base.css	chrome://treestyletab/skin/Linux-base.css	os=Linux

content/treestyletab/group.xul

@@ -1,5 +1,5 @@
 <?xml-stylesheet type="text/css" href="chrome://global/skin/"?>
-<?xml-stylesheet type="text/css" href="chrome://treestyletab/skin/group/group.css"?>
+<?xml-stylesheet type="text/css" href="chrome://treestyletab/skin/group.css"?>
 <?xml-stylesheet type="text/css" href="chrome://treestyletab/skin/pseudo-tree.css"?>
 <!DOCTYPE page SYSTEM "chrome://treestyletab/locale/treestyletab.dtd">
 <page xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"
@@ -36,7 +36,7 @@
 	</vbox>
 </stack>
 <link xmlns="http://www.w3.org/1999/xhtml" rel="icon"
-	href="chrome://treestyletab/skin/group/group-icon"/>
+	href="chrome://treestyletab-group/skin/group.png"/>
 <script type="application/javascript"><![CDATA[
 
 Components.utils.import('resource://treestyletab-modules/groupTab.js');

skin/classic/treestyletab/group.css

@@ -27,7 +27,7 @@
 }
 
 .icon {
-	list-style-image: url("chrome://treestyletab/skin/group/group-icon");
+	list-style-image: url("chrome://treestyletab-group/skin/group.png");
 /*
 	list-style-image: url("chrome://global/skin/icons/folder-item.png");
 	-moz-image-region: rect(0, 32px, 16px, 16px);