Document various undocumented attributes of QQuickItem

Document various undocumented attributes of QQuickItem

Add docs for various properties and methods that are public but weren't
documented. Also marked various things that shouldn't be documented as
\internal.

Newly documented attributes in C++ API include:
- x, y, z, implicitWidth/Height, baselineOffset
- transformOrigin, rotation, scale
- stackBefore(), stackAfter()
- mapTo*(), mapFrom*(), mapRectTo*(), mapRectFrom*()
- childItems(), childAt()
- updatePolish() and polish()
- state
- opacity, visible, enabled
- mouse, hover, wheel, key, touch and grab event handlers
- drag and drop event handlers
- acceptedMouseButtons(), filtersChildMouseEvents(),
  acceptHoverEvents(), grabMouse, ungrabMouse()
- focus, activeFocus, isFocusScope(), scopedFocusItem()
- methodInputEvent(), inputMethodQuery()
- Flag enum, setFlags(), flags()
- window()

Newly documented attributes in QML API include:
- smooth, antialiasing
- baselineOffset

Change-Id: If8596b2a488e6ae37a3eb1f7d6dd0911479e9c0e
Reviewed-by: Andrew den Exter <andrew.den-exter@nokia.com>