Add leaflet event to pyqtlet

[hari0603]

Request to add drawDeleted event similar to drawCreated event

[samhattangady]

Adding a Leaflet event as a pyqtSignal

To add a leaflet event as a signal, a few things need to be done.

1. Identify the correct leaflet event. For drawDeleted, it will probably be in leaflet-draw.
2. Identify parent object. In case of drawDeleted, the best bet would be to find where drawCreated is called, and create it along with that.

Once we have this, to actually implement a signal, is fairly straightforward. Base class pyqtlet/leaflet/core/evented.py has a _connectEventToSignal method that can be used to connect any leaflet event to a pyqtsignal.

So in order to create a new event/signal:

1. Create a pyqtSignal in the correct object.
2. Call a self._connectEventToSignal() in the objects __init__ method to connect the leaflet event to the pyqtSignal you just created.
3. Profit.