* Moved app::DocRange to view::Range
* Moved range_utils to view/cels.cpp
* Moved layer_utils to view/layers.cpp
* Created more functions in app::Site to access selected cels
(they use the functions from view/cels.h)
Added a new Events object with :on() and :off() methods to start or
stop listening to a specific event respectively. This also allows to
add several callbacks for the same event.
Replaced the temporal Site.onChange & Sprite.onChange implementations.
Related to several issues (enable more possibilities for): #138, #1403, #1949, #2965, #2980