more portable access to icon -- avoid Isabelle_System which is not yet initialized in bootstrap;

--- a/src/Pure/System/cygwin_init.scala	Sat Sep 07 11:28:30 2013 +0200
+++ b/src/Pure/System/cygwin_init.scala	Sat Sep 07 11:36:03 2013 +0200
@@ -25,7 +25,7 @@
   def main_frame(isabelle_home: String, continue: Int => Unit) = new MainFrame
   {
     title = "Isabelle system initialization"
-    iconImage = new ImageIcon(isabelle_home + "\\lib\\logo\\isabelle.gif").getImage
+    iconImage = GUI.isabelle_image()
 
     val layout_panel = new BorderPanel
     contents = layout_panel

--- a/src/Pure/System/gui.scala	Sat Sep 07 11:28:30 2013 +0200
+++ b/src/Pure/System/gui.scala	Sat Sep 07 11:36:03 2013 +0200
@@ -114,7 +114,7 @@
   /* icon */
 
   def isabelle_icon(): ImageIcon =
-    new ImageIcon(Isabelle_System.platform_path(Path.explode("~~/lib/logo/isabelle.gif")))
+    new ImageIcon(getClass.getClassLoader.getResource("isabelle/isabelle.gif"))
 
   def isabelle_image(): Image = isabelle_icon().getImage
 }

--- a/src/Pure/build-jars	Sat Sep 07 11:28:30 2013 +0200
+++ b/src/Pure/build-jars	Sat Sep 07 11:36:03 2013 +0200
@@ -232,6 +232,8 @@
   mkdir -p "$(dirname "$CHARSET_SERVICE")"
   echo isabelle.Isabelle_Charset_Provider > "$CHARSET_SERVICE"
 
+  cp "$ISABELLE_HOME/lib/logo/isabelle.gif" isabelle/.
+
   isabelle_jdk jar cfe "$(jvmpath "$TARGET")" isabelle.GUI_Setup META-INF isabelle || \
     fail "Failed to produce $TARGET"