- new file item_type_filters.dart for the hiddenItemTypes function.
- reduces code duplication in statistics_screen, categories_screen and base_library_tab_screen
The "Show extensions" Button was not working properly when categories were hidden.
An Exception would throw:
```
_AssertionError ('package:flutter/src/material/tab_controller.dart': Failed assertion: line 202 pos 12: 'value >= 0 && (value < length || length == 0)': is not true.)
```
And jump to the end of the tab list regardless of where you clicked the button.
- Extract localizedItemType() from `base_library_tab_screen.dart`
- Add localizedSources() and localizedExtensions() for browse_screen.dart
- Reduce if-statements in statistics_screen.dart
- Reduce if-statements in categories_screen.dart
- Reduce if-statements in browse_screen.dart