Do not apply changes in preference page multiple times

In case both the `Apply` and `Ok` button are pressed, all changes were
applied two times (and therefore no change happened at all).

org.scala-ide.sdt.core/src/org/scalaide/ui/internal/preferences/SaveActionsPreferencePage.scala

@@ -131,6 +131,7 @@ class SaveActionsPreferencePage extends PreferencePage with IWorkbenchPreference
       prefStore.setValue(saveAction.id, !previousValue)
     }
     prefStore.setValue(SaveActionExtensions.SaveActionTimeoutId, timeoutValue.getText())
+    changes = Set()
     super.performOk()
   }