Implement batter solution for navigating between application main pages

Top pulley menu solution was too limiting. Pulley menus are not supposed
to contain many items, but future versions of YTPlayer will have at
least 4 new top level pages.

The new solution uses similar approach to Jolla email application. The
top level pages are accompanied by a menu page reachable through left
swipe gesture.
YTListModel: Implement generic support for filtering content

This makes it possible to specify content filter in QML code instead of
hardcoding it in C++. This will be required by features dealing with
YouTube activities API.