Cast autosave delay preference to int

Oops. How long has this been in there, you think? At least 3 years.
As seen in #5729, when changing this preference it results in an error that it can't set the interval to a string. The default is an integer, so if you don't edit it it works fine, but if you edited the preferences file, it gets a string from the preferences file. Casting both to an int should work.

cura/AutoSave.py

@@ -1,4 +1,4 @@
-# Copyright (c) 2016 Ultimaker B.V.
+# Copyright (c) 2019 Ultimaker B.V.
 # Cura is released under the terms of the LGPLv3 or higher.
 
 from PyQt5.QtCore import QTimer
@@ -16,7 +16,7 @@ class AutoSave:
         self._application.getPreferences().addPreference("cura/autosave_delay", 1000 * 10)
 
         self._change_timer = QTimer()
-        self._change_timer.setInterval(self._application.getPreferences().getValue("cura/autosave_delay"))
+        self._change_timer.setInterval(int(self._application.getPreferences().getValue("cura/autosave_delay")))
         self._change_timer.setSingleShot(True)
 
         self._enabled = True