Avoiding setting default values to properties, grouped code a bit better.

- Added PageStackWindow and Page elements to exploit qtcomponents' screen orientation support.
To do that part of QML code had to be modified a bit because many function calls assumed that the "window" element was in their scope, which has always been the case so far because it was the root QML item, but not anymore, now that the root item is a PageStackWindow, and "window" has been moved as a child of the initialPage.

- A new Orientation Lock setting has been added in Fingerterm's menu. It lets you choose between Auto, Portrait Lock and Landscape Lock (defaults to Auto).