Use actions of ViewBox's contextMenu in full menu.

The main change is on `ViewBox.getContextMenus`, which now returns an
up-to-date of actions that `GraphicsScene.addParentContextMenus` can use.

Also, `getContextMenus` was given a default implementation in the base
class (falling back on `getMenu` if defined), and some cleanup was done.
fixed context menu handling for non-GraphicsItems