trigger writeback

[s-cork]

close #46

Implements a trigger('writeback') on all components.

It also removes the init method which was only used to create the trigger method and instead just adds trigger to Component.

It would mean that making a textbox writeback on change would be as simple as

from anvil_extras import augment

def textbox_change(self, sender, **event_args):
    sender.trigger('writeback')