Event System Improvements

[RobLoach]

@brianwatling following discussion here so we don't lose it. Thoughts on this? Goal is to consolidate most event systems into just one nk_console_event_handler* events.

• Make a enum nk_console_event_type with DESTROY, CHANGE, and CLICK
• Add a enum nk_console_event_type to nk_console_event_handler
• Have nk_console_event_handler* events in nk_console as a cvector for all events
• Move nk_console_button_data::onchange to the new events pattern
• Move nk_console::destroy to the new events pattern
• Keep render outside of the event system to keep performance a priority
• Re-work the invoke events to take a enum nk_console_event_type
• Rather than nk_console_onchange(widget), call nk_console_invoke(widget, NK_CONSOLE_EVENT_TYPE_CHANGE)... or something?
• Update nk_console_set_onchange_handler() to nk_console_add_event_handler()
• Update nk_console_set_onchange() to nk_console_add_event

[brianwatling]

I think your suggestion to have an enum for the event type seems reasonable. That'll help reduce the API surface for event handling.

Have nk_console_event_handler* events in nk_console as a cvector for all events

Are you suggesting all events are stored in a single array that's scanned when an event is triggered? Seems like a clean approach and performance shouldn't be a concern given events are triggered relatively infrequently - so I think this is decent way to implement this