MiniPauker Code

- ImportExportController and LearningController extracted out of UIController
- Introduced PaukerDisplay for centralizing view management
- In tasks, switched from prev/next Displayables to the use of ViewNavigation
- UIComponent now calls updateView itself before being displayed
- Repeat Expired/Unlearned highlighted when they are non-empty (works only partially, namely the highlighting disappears e.g. when the item is hovered over)
- Open / Import UI: refactoring for better readibility incl. replacement of item indexes with their labels in action handling