more robust Sledgehammer in Prover IDE;

--- a/NEWS	Thu Apr 19 14:59:17 2012 +0200
+++ b/NEWS	Thu Apr 19 15:02:13 2012 +0200
@@ -8,6 +8,8 @@
 
 * Prover IDE (PIDE) improvements:
 
+  - more robust Sledgehammer integration (as before the sledgehammer
+    command line needs to be typed into the source buffer)
   - markup for bound variables
   - markup for types of term variables (e.g. displayed as tooltips)
   - support for user-defined Isar commands within the running session