Add basic API for custom components

[j-piasecki]

Description

This PR adds a simple API for creating custom components:

• components with type Custom are treated like Stack when measured, and its children are positioned at the top-start.
• Custom function accepts additional argument with custom config
• custom config may be used to store data between updates
• custom config may additionally contain hooks for specific moments:
  • dropHandler - invoked when the node is dropped from the tree
  • createView - invoked when the view enters the view hierarchy, when implemented it must return a reference to the created view
  • overrideViewProps - invoked after the ViewManager finishes setting up the newly created view, it may be used to overwrite them
  • updateView - invoked during update, receives previous and the next node
  • deleteView - invoked when the view leaves the view hierarchy, when implemented it must delete the created view (note that this method may be called without calling dropHandler to reorder views)