core: use selected widget from main screen in getWidgetToSwitch: if we are not on "main" screen, selected_widget may not refer to a widget that we want to change. To avoid that, getWidgetToSwitch check and use `_selected_widget_main` if suitable. In other words, a widget from "main" screen will be returned and switched, even if we are on a other screen (e.g. if on "extra", if a widget like ShareWidget is shown).

-e git+https://github.com/kivy/kivy.git#egg=kivy
-e hg+https://repos.goffi.org/sat#egg=sat
-e .