add "interacting" read-only option

[ryanttb]

At any time, we should be able to tell if a user is interacting with the map so we can cancel things like downloading large images, etc.

[ryanttb]

Possibly complement it with interactbegin and interactend events.

interacting: false
* user clicks while mode == drawPoint
=> interactbegin event
interacting: true
=> interactend event
=> click event
=> shape event
interacting: false