Refactoring AssociatedFiles and AssociatedFilesUI

Considerable refactoring to remove code duplication in classes
AssociatedFiles and AssociatedFilesUI:

• From two different AssociateFilesUI::associateUrl functions down to one.
• Two functions of name AssociateFiles::associateDocumentURL refactored and split into four distinct functions that can be used separately (no more 'dryRun' flag).
• Adding automated tests in KBibTeXNetworkingTest covering some but not all cases.
• Adding/updating functions' comments in header files.
• Various code modernizations and improvements, such as preferring QUrl::isValid() over QUrl::isEmpty().