Merge `isLongPressedMangaStateProvider` and `isLongPressedStateProvider`
Both providers do the same thing and it is impossible to change the value of the provider in different places in the app at the same time.
So just use one `isLongPressedStateProvider`. Reduces code duplication and there is no confusion about which one to use.
Previously, the same select bar and button styles were defined in both
`library_screen.dart` and `manga_detail_view.dart`, resulting in repeated code.
This commit extracts the select bar and its buttons into reusable widgets
to reduce duplication and improve readability and maintainability.
Before fixing the Category Removal Bug (commit 58537281), the "Default" Category would be empty when removing a populated category.
So when there is only one Category left, and the user hides that one Category, `tabCount` would be <= 0, leading to an Exception:
```
_AssertionError ('package:flutter/src/material/tab_controller.dart': Failed assertion: line 116 pos 15: 'initialIndex >= 0 && (length == 0 || initialIndex < length)': is not true.)
```
Now there is a check (`tabCount <= 0`), that IF tabCount is ever <= 0, it would show `_bodyWithoutCategories`.
getMangaChapterDirectory() now accepts an optional mangaMainDirectory
to avoid calling getMangaMainDirectory() and thus getDirectory() twice in places where both methods are called successively.
Added a checkbox "ignoreFiltersOnSearch" to the library search, so when checked, the whole library will be searched, not just the filtered library (Downloaded, Unwatched/Unread, Started, Bookmarked)
+ added some comments
