Fix recentlyAudible Widgets and Quick API.

Rename all uses of wasRecentlyAudible to recentlyAudible.
Add missing recentlyAudible properties.
Change QtQuick slots to simple functions.
Change affected demobrowser example.
Adjust documentation for the API.

Change-Id: I5a6f7b8384c0b7e34afaa5c412a5543c210d3ef9
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@theqtcompany.com>
Reviewed-by: Michal Klocek <michal.klocek@theqtcompany.com>