Inter-element communication: implement action threading using zippers

[ognen]

Currently, if an element wishes to communicate with another element it issues a 'global' action (starts with '.', which will hit the first ancestor element in the ancestors chain that has an update for this action defined.

Introduce new way of threading actions.

• elUpdates.thread(action, movementOrSelector, targetAction)
• this is interpreted as an instruction to thread the current action to another location or locations
• the movementOrSelector parameter can be
  • a zipper movement function that is used to locate the target element from the current element's locations
  • a selector (zip.select) which can be used to select one or more elements (starting from the current element's location)
  • targetAction is the the action to be triggered on each of the target elements. It is optional, and if not provided it is the same as action
• error handling: when threading, if the action is not found on the target element, an Error is thrown
• elUpdates.threadToFirst(action, selector, targetAction)
• same as thread except that the action is attempted on each element defined in the selector until a recipient is found in which case the threading stops
• deprecate "global" actions (starting with a .)
• reimplement "global" actions using threadToFirst

[ognen]

Depends on #53 for the select function.