Unify naming for library / catalog

[BPerlakiH]

I think for a better user experience we should unify the naming in the app.
Currently we are using "catalog" and "library" interchangeably. We should communicate with our users in a consistent way.

Probably this screenshot describes the level of confusion around the naming, taken from macOS,
showing both the welcome screen and the settings at the same time:

[kelson42]

@BPerlakiH After thinking twice I believe:

• The catalog is the list of books metadatas
• The (online) library is the list of books (with metadata)

I challenge therefore the idea that both words have the same meaninh.

Here we fetch the online library catalog.

[kelson42]

@BPerlakiH Unsure what should be done next here? Anything? What dou you think?

[BPerlakiH]

I would suggest to change in most places the wording from "Library" to catalog.
We have a couple of very obvious cases, such as:

On second screen shot (settings), it is also a bit confusing, as we really filter the entries in the catalog by user selected language(s), so these 2 sections Library and Catalog really referring to the same thing, I would say.

[BPerlakiH]

Maybe download (using cellular) is really part of getting the book from the library, but the languages could go under the Catalog.

[kelson42]

I'm unsure about the solution to implement. Postponing.