Qt: make ground for proper main interface size management

Qt: make ground for proper main interface size management

- Simplify switching between widgets in central stack: showTab() allows
switching only between two states - video state and playlist state - and
does everything needed according to circumstances

- Do not setMinimumWidth/Height/Size() on main interface, because it
inhibits children sizeHint() evaluation, thus preventing interface
self-adjustment when controls are added/removed or shown/hidden.

- Subclass QStackedWidget so it's minimumSizeHint() matches exactly
the one of the currentWidget(), and not the sum of all children.

- Suggest compact mode implementation (commented out for now):
setSizeConstraint( QLayout::SetFixedSize ) on main interface layout.
This automatically keeps and adjusts fixed size in respect to
sizeHint() of children.