Adapted some syntax to PEP8
Modified the ListWindow and the OptionsListViewWidget to recreate the liststore only when necesary (issue #123).

* Removed the activate method from the ListWindow and moved the relevant code to other places to help with the flow of the OptionsWidget.
* Correctly implemented the update_options and update_current_key callbacks to do what they should (recreate the list and select the currently selected item respectively)
* TODO: maybe the ListWindow should be merged with OptionListViewWidget? Right now they seem like two layers of the same thing, maybe it could be simplified by merging.
* TODO: the "mouse" flag for the window position seems to put the center of the windo on the mouse, instead of the corner (like the poups). Should look into that.