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.