Cleanup how the pages and media actions are implemented.

This moves some of the logic to the new RingQtQuick.CallBuilder C++
object. It also group all the page handling and media actions into
a new BasicView.PageManager module.

Finally, it also refactor some business logic into the "main" QML
file (BasicView.Basic). This was done to have all the workflow
related configuration into a single, well commented, file.