CardView: rework the drag'n'drop functionality.

 * simplified SwipeArea and SlidingItemArea
 * use a SwipeableCard based on ListView to swipe out the cards; this enables us to use a MouseArea on top of tag to drive the drag
 * the drag is now started with a simple long press, no need to release the mouse button to begin the drag
 * Use VisualDataModel components for the groups and for the cards to have a more flexible way to handle them in the view
 * Use a PathView for the group of cards instead of a Repeater, to be able to better control the way the cards are positioned
 * The MouseArea for driving Drag'n'Drop is place in the CardWindowWrapper, as this component never gets deleted when a delegate disappears
 * When the drag begins, the card is reparented to a top-level item which also contains all the Drop logic
 * It is possible to drop the card anywhere in a group of cards, or between groups.
 * By dragging a card of the sides of the screen, the list of cards is scrolled automatically.

Sometimes the drag'n'drop bugs, and the result can be a crash of luna-next. This is being worked on.

Signed-off-by: Christophe Chapuis <chris.chapuis@gmail.com>