digital_electricity: save and load configurations

* Used GCComboBox instead of GCDialogBox
* Previous configuration is saved and loaded on start
* changed variant -> var
* Removed "totalComponents" which is obtained by imageName.length
* removed "changeMode()" the tutorial level is left to the previous
  level when changed from Tutorial Mode -> Free -> Tutorial Mode

Signed-off-by: Rudra Nil Basu <rudra.nil.basu.1996@gmail.com>