As we still create a `Reader` in the deprecated code of `Library`,
we need a way to create a reader without raising a deprecated warning.
So we create a another constructor with a dummy argument and we use it.
As the `Entry` is still created by `Reader` we need a way to create a
entry without raising a deprecated warning.
To do so we create a second constructor with a dummy argument.
This second constructor is private and is not marked as deprecated so we
can use it.
kiwix::fileExists only checks for file existence now
kiwix::fileReadable will check if the file is readable (implicitly checking for file existence also)
Library became thread-safe with the exception of `getBookById()`
and `getBookByPath()` methods - thread safety in those accessors is
rendered meaningless by their return type (they return a reference
to a book which can be removed any time later by another thread).
Introducing a mutex in `Library` necessitates manually implementing the
move constructor and assignment operator. It's better to still delegate
that work to the compiler to eliminate any possibility of bugs when new
data members are added to `Library`. The trick is to move the data into
an auxiliary class `LibraryBase` and derive `Library` from it.
Originally `LibraryManipulator` was an abstract class completely decoupled
from `Library`. Its `addBookToLibrary()` and `addBookmarkToLibrary()`
methods could be defined in an arbitrary way. Now `LibraryManipulator` has to be
bound to a library object, those methods are no longer virtual, they always
update the library and allow for some additional actions via virtual
functions `bookWasAddedToLibrary()` and `bookmarkWasAddedToLibrary()`.
