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).

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4233be74572a1af8eebd502c009a10641c63ca0c 0.7.0a3
5c28cf8815319abd8c963022091cf4726764a3e6 0.7.0b1
51f73d210da908c9fb292e6bd7fda42a9d2b8453 0.7.0