Pass js function as prop to native view

[nick-thompson]

This will be some duktape/juce trickery in readVarFromDukStack inside the EcmascriptEngine implementation to check if the given stack index is a function (duk_require_function or similar), and if it is, push that function into some global stash keyed by a unique id (https://wiki.duktape.org/howtonativepersistentreferences), then construct a juce::var::NativeFunction that captures that unique ID and invokes that function from the global stash in the js environment when the constructed native function is invoked.

This will enable cleaner View callbacks (i.e. can get rid of the dispatchViewEvent hack), which will facilitate interesting components like a Canvas component who passes an onDraw function prop to native to be called back by a timer every 30-60Hz