Merge branch '15.06'

* 15.06:
  Bump version to 15.05.93
  Add a changelog file
  add missing width and height to section icon
  add missing color to tool_button
  add setting for sidebar background color
  Make cura_app executable
  Move Sidebar so it is on top of everything else
  Do not slice if there is any setting with an error value
  Start adding some more proper error/warning values to settings
  Position the setting tooltip to the left of the settings instead of below
  Set version to 15.05.92

plugins/CuraEngineBackend/CuraEngineBackend.py

@@ -110,6 +110,9 @@ class CuraEngineBackend(Backend):
         if not objects:
             return #No point in slicing an empty build plate
 
+        if kwargs.get("settings", self._settings).hasErrorValue():
+            return #No slicing if we have error values since those are by definition illegal values.
+
         self._slicing = True
         self.slicingStarted.emit()