Merge branch '15.06'

* 15.06:
  Update changelog
  Correct the bottom offset we add when setting the volume for scale to max
  Display progress information during processing of layer data
  If findObject returns none but object_id != 0 use the selected object
  Offset the displayed rotation angle so it does not overlap the mouse cursor
  Abort attempts to connect if an error is thrown when connecting to the serial port
  Fix recent files on Windows
  Defer opening the webbrowser until the next run of the event loop
  Disable slicing and platform physics when an operation is being performed
  Rework LayerData mesh generation for improved performance
  Performance: Only calculate the platform center once, not for every poly
  Add application icons for all three platforms

plugins/USBPrinting/PrinterConnection.py

@@ -173,6 +173,10 @@ class PrinterConnection(SignalEmitter):
             Logger.log("i", "Could not establish connection on %s: %s. Device is not arduino based." %(self._serial_port,str(e)))
         except Exception as e:
             Logger.log("i", "Could not establish connection on %s, unknown reasons.  Device is not arduino based." % self._serial_port)
+
+        if not self._serial or not programmer.serial:
+            self._is_connecting = False
+            return
         
         # If the programmer connected, we know its an atmega based version. Not all that usefull, but it does give some debugging information.
         for baud_rate in self._getBaudrateList(): # Cycle all baud rates (auto detect)