Draft version, this commit will amend later.

It contains the definition of some chart event interfaces which a gadget want to response a certain event occurred in a chart. Chart gadget will call this interfaces to pass parameters. Then gadget do some custom processing inside this function.

For chart introspection, chart gadget should provide the introspection interfaces. Then other gadgets can acquire this function to get internal value for a certain chart item by specify the chart id and item name.